Technical Field

The present invention concerns improvements in or relating to optical communications systems, as also user terminal equipment for use in such systems. A typical communications system comprises:- a transmitter terminal, usually a laser source and modulator; a fibre link for propagating modulated transmitted optical signal; and, a receiver terminal, usually including an optical detector and demodulator.

For many shorter-haul applications of fibre optics it is very desirable to minimise costs, particularly for the terminal equipment. For this reason bidirectional transmission on a single fibre is desirable. Furthermore the cost per of optical power is very low for a laser but the cost per device is comparatively high, so that a method for spreading the devices cost between terminals is desirable.

Background Art

In an optical communications system allowing two-way

SUBSTITUTESHEET

flow of information between users, it is conventional practice that each user is provided with both a receiver and a transmitter, the transmitter of one user being linked to the receiver of another by means of an optical fibre. Where each user generates a distinctive modulated signal, it is possible to link all users by means of a common optical fibre.

Recent publications on this topic include the following:- Hall, R.D., et al, "Bidirectional transmission over 11 km of single-mode optical fibre at 34m bit/s using 1. 3 ^u-m

LEDs and directional couplers," Electron Lett, 1985 vol. 21 pp. 628-629; and, Stern, M, et al, "bidirectional LED transmission or single-mode fibre in the 1300 and 1500 nm wavelength regions," Electron Lett, 1985 vol. 21, pp. 928-929.

Since in the system aforesaid each user provides- - power for generating its own distinctive modulated signal, the supply and maintenance cost for each user terminal is relatively high, as also is the running cost. An alternative approach to bidirectional transmission, one relying upon the use of a pair of optical link fibres, one fibre for forward transmission and the other for return, has recently been discussed. See Cheng. S.C., et al, "A distributed star network architecture for interoffice applications, "Tech. digest IOOC-ECOC, vol

SUBSTITUTESHEET

1, Venice, Oct 1985 pp 699-702. The system configuration there discussed is as shown in figure 1. The transmission signal for this system is provided by two lasers TX _] _, TX ₂ each operating at a different optical wavelength passes through a first modulator Ml and is combined with the other signal component ₂ ky means of a wavelength division multiplex (WDM) multiplexer MX. The combined signal is then distributed to a number of users T2 via an energy splitter SR and corresponding single-fibre forward-direction links F _j _. At the receiver end T2, the transmitted signal is demultiplexed DX and the modulated component Xj is directed to the detector of the receiver RX ₂ « The other signal component \2 ^- ^s directed to a second modulator M2 and thence returned by a further link fibre F2 and combiner CR to the receiving detector RX _j _ of the transmitter terminal Tl. It is noted that in this configuration the modulator M2 of the user operates upon carrier signal supplied not locally, but from the transmitter Tl, a remote facility. Since the number of lasers required for multi-user systems can be reduced to just the two provided at the transmitter terminal, system costs can be much reduced and problems of laser reliability and maintenance minimised. Notwithstanding, two fibre links, forward and return path, need be provided for each user.

SUBSTITUTE ! ?_J __ <C"

Disclosure of the Invention

The present invention is intended to provide an alternative and relatively low cost optical communications system. It is also intended to provide user terminals of less complexity and of inexpensive design.

In accordance with a first aspect of this invention thus there is provided an optical communications system comprising:- a transmitter including a light source; an optical fibre link interfaced to said transmitter, for propagating light signal; and, at least one user terminal remote from said transmitter, said one user terminal being interfaced to said fibre-link to receive said light signal; wherein, said one user terminal includes:- a modulator for applying a modulation to said light signal; and, signal return means, co-operative with both said modulator and said fibre link, for returning to same said fibre- link, optical signal including said applied modulation.

It will be noted that in the optical communications system disclosed, modulated signal is produced by the user using signal supplied from a remote transmitter source. The user thus has no longer any need for an individua3 light

light source, and in consequence the user terminal can be much simplified.

It is noted that the transmitted signal itself also may be modulated at source. In particular, the invention as embodied may thus comprise:- a transmitter, including a light source and a first modulator, for transmitting a light signal including a first modulation; an optical fibre link, interfaced to said transmitter, for propagating modulated light signal; and, at least one user terminal, remote from said transmitter, said one user terminal being interfaced to said fibre link to receive said modulated light signal; wherein, said one user terminal includes:- a second modulator for applying a second and distinguishable modulation to said modulated light signal; an optical detector, responsive to light signal including said first modulation; and, signal return means, co-operative with both said second modulator and said fibre link for returning light signal including said second modulation to said fibre link, and for passing optical signal including said first modulation to said optical detector.

It is noted that the user terminal modulator and optical detector may be placed in series. In this case said signal return means may comprise a ref3ector located

between the modulator and the detector - for example, a partially reflecting mirror or wavelength reflective mirror as may be appropriate. Alternatively, the user terminal modulator and optical detector may be coupled in parallel. In such latter case each may be preceded by a common coupler with a reflector - eg. a mirror, located following the modulator. In both cases just mentioned the light signal is returned via the modulator. Since then light signal passes twice through this modulator, modulation is applied twice. For data rates considered here, and provided the transit delay between the modulator and reflector is minimal, modulation pulse broadening will be inappreciable. To advantage, the overall result will be an increase in the modulation depth, ie. this depth will be doubled. The resultant modulation depth is thus produced using significantly less power than would be necessary to produce the same modulation for a single pass configuration.

It is further noted that the transmitter and more than one user terminal may be connected by corresponding single optic fibres, and the user terminal interfaced to these corresponding fibres, in each case, to provide for bidirectional transmission between the transmitter and each user terminal. Where, as in the embodiment just described, user

modulation is applied to the source modulated signal, it is important that the first and second modulations are distinguishable. Several variants are possible, for example:- i) The source may provide light signal at two distinguishable optical wavelengths, first modulation being applied to light of one wavelength and second modulation being applied to light of the other; ii) The source may provide light signal at a unique optical wavelength, different modulation bands being adopted for first and second modulation; iii) One of the first and second modulations may be amplitude modulation whilst the other may be phase modulation; iv) The source may provide light signal having two orthogonal polarisations, first and second modulation being applied to respective polarisations; or, v) Time-division multiplexing may be adopted.

Modulators for phase, amplitude, and or polarisation, may be for example of fibre type (see UK Patent

Application No: GB 2170016, integrated-optic type, or of pico-optic type (see UK Patent Application No: GB 2147115. The reflector may be embodied in any of several ways. Thus it may be:- i) A mirror integrated with tr.e modulator;

ii) A mirror integrated with the detector; iii) Provided by the detector itself, surface reflection being utilised; iv) Of fibre loop type; or, v) Of fibre type, with a coupler for signal extraction. The detector may be of semiconductor silicon or III-V compound type and for convenience may be integrated with post-detection circuit components. Alternatively, it may be part of a coherent system. This latter is particularly desirable for the case of phase modulation.

In accordance with a second aspect of the present invention there is provided a user terminal comprising essentially:- a modulator, for applying modulation to a received optical signal; an optical detector; and, signal return means, co-operative with said modulator and an optical fibre input to said user terminal for returning there to optical signal including said applied modulation.

Brief Introduction of the Drawings

In the drawings accompanying this specification:- Figure 1 is a block scher'itic drawing showing the

configuration of a known communications system, a system having two fibre links one for forward transmitting the other for return signal;

Figures 2 and 3 are block schematics showing two variants of a communications system embodied in accordance with the present invention;

Figure 4 is a block schematic drawing of a preferred configuration, also embodied in accordance with the present invention; and, Figure 5 is a plan drawing showing the detailed arrangement of the modulator, suitable for the system configuration of the preceding figure.

Description of Embodiments

So that the invention may be better understood, embodiments of the invention will now be described, by way of example only, with reference to figures 2 to 5 of the accompanying drawings.

With reference to Figure 2, there is shown an optical communications system comprising a source terminal Tl and a user terminal T2, interconnected by means of a single optical fibre-link F. At the user terminal T2 light from the optical fibre-link F is interfaced to a modulator M and, following this modulator, light signal is passed to a

receiver detector RX ia an interposed mirror member

SD, which latter serves to provide signal division as will be discussed below.

In this example the source terminal Tl includes a transmitter TX as also a receiver RX^. The latter receiver RX^ is arranged to receive light coupled out of the fibre-link F by means of a coupler C. The light signal that is received is directed from the signal division mirror SD of the user terminal. In this example the transmitted light signal is modulated at source and further and distinguishable modulation is applied to the transmitted signal by the user modulator M. The mirror SD serves to pass light signal including the first modulation to the user receiver ^R X2* ^{At the same} time it serves to reflect light signal including the user applied modulation. It thus affords bidirectional transmission between the -source terminal and the user.

It will be noted that a complete division of signal including only user defined modulation, from signal including only source defined modulation, is not requisite. Each receiver RX^ , RX could be preceded by a suitable filter, polariser or other modulated signal discriminating device, as appropriate, or could be tuned to detect sic _al only of the appropriate modulation,

depending on implimentation. However, given a dual wavelength source of light signal, it is convenient to perform source and user modulation at each wavelength, respectively, and to use as mirror SD a low-pass or high- pass wavelength selective filter, as appropriate, to effect adequate separation of the modulations. Light signal reflected from this mirror SD is then directed back towards the source terminal Tl and coupled to the source, receiver RX^ by means of the optical coupler C. A coherent detection arrangement is shown in Figure 3 a variant of this communication system. A reference signal is extracted from the main fibre-link F by means of the optical coupler C and following reflection at a mirror R this reference signal is directed back to the source receiver RX^ where it can be used for coherent detection of the user modulated signal. This allows sensitivity gain and can, if another coupler is provided, afford discrimination against amplitude modulation in favour of phase. As a precautionary measure, an isolator I may be interposed between the transmitter TX and the coupler C. A preferred configuration of the system is shown in figure 4. In this system a laser source TX directly modulatec at 565 Mbit/s, provides data transmission in one directiori. At the other end of the link F, a portion of this siς-.al is remodulated at 34 Mbit/s in a reflective

electro-optic modulator M to provide the return data stream. This technique demands the use of a coding format for the higher-data-rate channel that does not permit extended intervals of zero light tensity. Here a 1.3Atm laser transmitter module TX is fusion-spliced to one port of a taper-fused fibre coupler C _} with 1:1 power coupling ratio. A 3.6 km length of loose-tubed standard monomode fibre was spliced to one output port of this coupler C^ and to one input port of a second taper-fused coupler C . One output of this coupler C was spliced to a 565 Mbit/s PINFET receiver RX ₂ , and the other to a fibre polarisation controller P spliced to the reflective modulator M.

The modulator M shown is a titanium-diffused LiNbθ3 directional coupler employing uniform p _> electrodes E of length 15 mm. The input fibre F was mounted in a silicon V-groove support chip S and attached to the modulator M as shown in Fig. 5. A reflector R was simply formed by gilding the polished end of a short length of fibre FF and locating this against one outport port of the modulator M. The coded 34 Mbit/s signal was supplied to the modulator M at up to 12 V peak, superposed on a small DC bias voltage establishing the operating point. Although the modulator response is sensitive to polarisation, adjustment of the pcLarisation controller P is found to be required only

infrequently following initial optimisation.

After passing back along the link, the remodulated signal is detected by a 34 Mbit/s PINFET receiver RX _j _ spliced to the first fibre coupler C _j _. The high-frequency content of this signal due to the original 565 Mbit/s modulation is integrated out by this receiver RX _j _, leaving only the 34 Mbit/s data output B.

The performance of this system has been evaluated for various power splitting ratios for the second coupler C ₂ . In Table 1 the results presented were obtained for a coupler of ratio 1:9, with the larger power fraction in the modulator arm. With a launched power of - 3dBm, 10~9 bit error rate (BER) is found in both transmission directions simultaneously with available system margins of 16 dB in the 565 Mbit/s channel and 27 dB in the 34 Mbit/s channel. No discernible difference is detectable in the received 565 Mbit/s waveform with and without the 34 Mbit/s drive signal applied to the modulator M. The major portion of the excess losses is associated with imperfect demountable and fused splices and can be substantially reduced. It is to be noted that the unequal margins are largely attributable to the highly asymmetric power coupling ratio in this case. By substituting a 1:4 coupler margins of 19 and 24 dB, respectively, with the same excess losses, and with a 1:1 coupler 23 and 22 dB,

SUBSTITUTESHEET

respectively, are found,

Table 1 SYSTEM PERFORMANCE WITH 1:9 COUPLER

565 Mbit/s 34 Mbit/s channel channel

Launched power -3 -3

(mean, NRZ coding), dBm Power division losses, dB 13 7 Excess losses, dB 6 13

Received power -22 -23 (10~ ⁹ BER) , dB

Receiver sensitivity -38 -50 (10 ^~9 BER) , dBm

System margin, dB 16 27

Power returned to laser, dBm -23

The use of 1:9 coupler yields an additional signigicant result. For this case maximum power is returned to the laser TX, and 10~ ⁹ BER performance can be maintained without the need for an optical isolator with -23dBm returned power. Only when the returned power is increased beyond this figure is there a marked degradation in BER accompanying reflection-induced instability in the laser TX.

It is clear from the system margins available that there is considerable scope for further resource sharing by utilising a single laser TX to supply many bidirectional links of this type. Although only relatively high and low data rates in the two transmission directions have been considered above, with application to the subscriber access connection in mind, the system concept is not restricted to asymmetric data rates, and alternative coding and modulation formats may be employed. The component requirements at the subsciber terminal are amenable to integration.