Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1991-11-14
1993-12-21
Chilcot, Jr., Richard E.
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359132, 359191, 358 86, H04J 1402
Patent
active
052725561
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to optical networks, and particularly but not exclusively to broadband passive optical networks (BPON) for carrying high definition television (HDTV) signals.
It is well known to use a BPON having a branched configuration to convey suitably signals from a head-end station to a number of customer stations. The use of such a system has been proposed for carrying standard definition PAL television. The proposed technical standards for BPONs require a maximum head-end to customer power loss of 26 dB. With a zero dBm launch power, this power budget is adequate to enable up to 32 channels of standard definition PAL to be carried using standard technology. HDTV however requires a far higher bandwidth than conventional television channels and so it is possible to fit only a few channels within the power budget of a conventional BPON. Along with the developments in technical standards associated with the switch from PAL to HDTV there is expected to be a proliferation in the number of channels available to the viewer. Accordingly an optical system capable of carrying fewer than, say, 16 channels is likely to be commercially unacceptable to cable network operators.
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.
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 a 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.
According to the present invention a system for distributing a plurality of channels from a head station to a number of customer stations comprises;
a head station arranged to provide a plurality of output channels at respective frequencies, a plurality of customer stations, and a broadband passive branched optical network coupled to the output of the head station and arranged to distribute the output channels from the head station to the customer stations, each customer station including a coherent receiver having a local oscillator laser and arranged to demodulate a selected one of the channels: characterised in that the head station comprises a head-end optical source having a frequency f.sub.0 ; and an associated modulator arranged to receive light from the optical source and to modulate the light with a sub-carrier multiplex of n channels, where n is a integer greater than 2, the n channels having frequencies f.sub.1, f.sub.2 . . . f.sub.n separated in frequency-space
REFERENCES:
patent: 4726010 (1988-02-01), Ali et al.
patent: 4783852 (1988-11-01), Auracher
ECOC 87, Technical Digest, vol. II, R. Olshansky: "RF multiplexing techniques applied to video distribution in local networks", pp. 122-125.
ECOC 87, Technical Digest, vol. III, C. Baack et al: "Coherent multicarrier techniques in future broadband communication networks", pp. 79-87.
Electronics Letters, vol. 24, No. 22, Oct. 27, 1988 (Stevenage, Herts., GB) W. I. Way et al: "Multichannel FM video transmission using travelling wave laser amplifier in 1300 nm subcarrier multiplexed optical system", pp. 1370-1372.
IEEE Journal on Selected Areas in Communications, vol. 6, No. 7, Aug. 1988, IEEE, (New York, US), R. Kishomoto et al: "Fiber-optic digital video distribution system for high-definition television signals using laser diode optical switch", pp. 1079-1086.
Patent Abstracts of Japan, vol. 9, No. 75 (E-306)(1798), Apr. 4, 1985, and JP. A, 59208952 (Fijitsu K.K.) Nov. 27, 1984.
Cook Andrew R. J.
Faulkner David W.
British Telecommunications
Chilcot Jr. Richard E.
Negash K.
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