Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1991-11-18
1993-09-14
Coles, Sr., Edward L.
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359132, 359191, H04J 1402
Patent
active
052454592
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to optical communications systems, and particularly but not exclusively, to systems employing a broadband passive optical network (BPON) for carrying interactive high definition television (HDTV) services.
Advancing technical standards for television signals and consumer-demand for an increasing range of associated services has led to a need for systems capable of carrying interactive HDTV. For the distribution of conventional PAL encoded television signals the use of a BPON having a branched configuration has been proposed. However, it is found that because of the increased bandwidth needed for HDTV signals conventional networks are unable to carry more than a few channels within the limits imposed by the available power budget. This problem is further exacerbated by the need in an interactive HDTV system to provide channels for return data transmission from the customer stations to the head-end station.
There is known from, ECOC 87, Technical Digest, Vol. III, pages 79-87; C. Baack et al.: "Coherent multicarrier techniques in future broadband communication networks" a system for providing ISDN services, broadband communication service, video telephone and broadband distribution services such as HDTV to customers via respective single fibres. The head station employs a separate laser for each wavelength to be transmitted onto the fibre network and the customer's station employs heterodyne detection. Upstream transmission from customer's station is obtained by feeding the output of a separate opto-electronic converter into a fibre coupler in the customer's station and, for each customer, feeding an output from a respective coupler at the head station to a respective opto-electronic converter.
In Japanese patent application No. 58-81812 (publication No. 59-208952) in the name of Fujitsu K. K. each of the two stations linked by go and return fibres has a local oscillator laser employed in a coherent heterodyne receiver. Light from the local oscillator laser in one station is modulated by local data for transmission to the other station. In such a point to point arrangement there is no need for a stable laser frequency because the feedback control acts to stabilise the frequency difference between the transmitter laser and local oscillator laser in a station.
The Fujitsu system is limited to one station receiving only f.sub.2 from a single remote station, from which it produces f.sub.1 -f.sub.2 and thereby controls the transmitter to maintain f.sub.1 -f.sub.2 constant. If a station were to be coupled to more than one remote station then the feedback control would have present the signals f.sub.1 -f.sub.2, f.sub.1 -f.sub.3, etc and would not be able to function.
There is disclosed in ECOC 87, Technical Digest, Vol II, pages 122-125, R. Olshansky: "RF multiplexing techniques applied to video distribution in local networks" a system employing sub-carrier multiplexing in a head station for the distribution of video signals over an optical network.
In the Olshansky system a number of voltage controlled oscillators operating at microwave frequencies are directly frequency modulated. The outputs from the oscillators are combined in a combiner whose output is used to intensity modulate the transmitter laser, and Olshansky refers to wide bandwidth (15 GHz) devices as detectors in the remote terminals. With the Olshansky head-end arrangement, it would not be possible to employ a coherent heterodyne detection system in the customer stations because of a characteristic of the transmission spectrum of such a head-end known by engineers as "chirp".
This problem is avoided in the system of the present invention by the use of the external modulator which receives the light from the head-end laser and modulates it with the transmission signal.
According to the present invention an optical communications system comprises:
a head-end station including an optical source, a bi-directional transmissive broadband passive branched optical network arranged to distribute signals from the head-end station
REFERENCES:
patent: 4726010 (1988-02-01), Ali et al.
ECOC 87, Technical Digest, vol. III, 1987, C. Baack et al: "Coherent multicarrier techniques in future broadband communication networks", pp. 79-87.
Patent Abstracts of Japan, vol. 9, No. 75 (E-306) (1798), 4 Apr. 1985, & JP, A, 59208952 (Fujitsu K.K.) 27 Nov. 1984.
ECOC 87, Technical Digest, vol. II, 1987, R. Olshansky et al: "RF multiplexing techniques applied to video distribution in local networks", pp. 122-125.
IEEE Journal on Selected Areas in Communications, vol. 6, No. 7, Aug. 1988, IEEE, (New York, U.S.): R. Kishimoto et al: "Fiber-optic digital video distribution system for high-definition television signals using laser diode optical switch", pp. 1079-1086.
Cook Andrew R. J.
Faulkner David W.
British Telecommunications public limited company
Coles Sr. Edward L.
Negash Kinfe-Michael
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