Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1993-03-15
1994-04-19
Chilcot, Jr., Richard E.
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359145, 359113, H04B 1024, H04B 1000
Patent
active
053051335
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to an optical communications system including at least one transceiver arranged to provide two-way communication over an optical communications channel, and to a transceiver for use in such a system.
Description of Related Art
Optical communications systems, generally using silica-based optical fibres for transmission of infrared light in the wavelength range 0.72 to 1.55 .mu.m, are becoming increasingly widely used. Thus far, fibre-based systems have predominantly been used in heavy traffic, trunk telephony applications where the fibres' large bandwidth justifies the expense of the optical terminal equipment.
Whilst optical fibres are increasingly being used in lower traffic applications, in such applications the high cost of transmitters and receivers is a limiting factor. In particular, the high cost of providing optical transmitters and receivers is one of the major obstacles preventing the widespread provision of fibre links to subscribers premises.
Consequently, there exists a need for less expensive equipment for the transmission and reception of optical signals.
Our copending British patent application 8916939.5 relates to an optical communications system including a pair of transceivers interconnected by an optical fibre. Each transceiver is arranged to provide two-way communication over the optical fibre, and each transceiver comprises a semiconductor light source such as a laser and a photodetector. Each laser provides optical signals for transmission via the optical fibre, and each photodetector detects optical signals received via the optical fibre. A respective filter, which is substantially opaque at the operative wavelength of the associated laser, is located between that laser and its photodetector.
This type of transceiver separates the send and receive signals after the incoming signal has passed through the laser. This technique, which is termed "optical feedthrough", enables bi-directional links to be configured without the need for line side components such as couplers, thereby reducing costs and network complexity. Thus, the laser die of a typical laser package can be aligned with a single mode fibre, allowing the transmitted signal to be efficiently coupled. A large area photodetector can be mounted some distance behind the laser to collect and monitor the optical output from the back facet of the laser. An optical signal transmitted into the laser package via the fibre will cause the laser to form part of the received signal path. If the photon energy of the incoming signal is less than the band gap energy of the laser, little absorption occurs, and the light is guided through the laser to be received at the rear-mounted photodetector. Conversely, if the incoming signal has a photon energy greater than the band gap of the laser, absorption would be expected to take place. However, it has been found that, even in this second case (for example for 1300 nm light passing through a 1550 nm laser), significant levels of incoming signal are present at the photodetector. This effect is due to the incoming signal "by-passing" the laser die, the level of signal received being related to the physical geometry of the transceiver package and the divergent nature of the light exiting the fibre. If the level of incoming signal detected by the photodetector is of sufficient optical power this component can be used to create an integral receiver.
It will be appreciated that there will be large levels of crosstalk present at the photodetectors due to the close proximity of the lasers. The method of avoiding unacceptable crosstalk described in the specification of our corresponding British patent application 8916939.5 is to operate in a WDM mode, the two transceivers having different wavelengths, with optical band pass filters being placed between lasers and their photodetectors.
The introduction of optical band pass filters between the lasers and photodetectors separates the send and receive signals, so that a bi-direct
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Bickers Lawrence
Cooper Anthony J.
Hunwicks Andrew R.
Barares Rafael
British Telecommunications public limited company
Chilcot Jr. Richard E.
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