Optical waveguides – With optical coupler – Movable coupler
Patent
1992-10-06
1994-04-19
Gonzalez, Frank
Optical waveguides
With optical coupler
Movable coupler
385 18, 385 22, 372 6, 372 94, G02B 626
Patent
active
053054024
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to optical filters and is of particular, but not exclusive, application to passive optical network transmission systems.
2. Related Art
One approach to providing communications links between stations is to employ a passive optical network between a central station, or exchange, and the stations.
A potential strategy for the evolution of passive optical splitter networks providing customer access in the local loop is as follows. Telephony and low data-rate services could initially be provided over a fibre-lean passive optical network (TPON) at a single operating wavelength, such as 1300 nm. Broadband services could then be added in two separate stages. Firstly, coarse-grained wavelength division multiplexing (WDM) could provide a few wavelength channels in the 1300 nm window: 20 nm spacing has been demonstrated, but a closer spacing of perhaps 10 nm is quite feasible. Just three additional wavelengths, 1280 nm, 1320 nm and 1340 nm, would be sufficient to provide a range of early broadband services such as ATM-based high-speed data, 32-channel broadcast digital video and 32-channel dedicated (switched) video. A fourth wavelength might be used to provide several HDTV channels. This approach would leave the 1500 nm window free for high density WDM (HDWDM) to be used in the longer term, when large numbers of channels are needed, in combination perhaps with the use of erbium-doped fibre amplifiers, or to provide two-way broadband customer services by wavelength multiplexing in the upstream direction. A nominal wavelength spacing of 1 nm has been demonstrated for HDWDM.
A major difficulty, however, of employing tunable lasers at customers' premises is that of accurate referencing of the laser wavelengths, particularly for HDWDM, where channels may be spaced as close as 1 nm or less. For broadcast services such as cable TV, tunable filters are key components for selecting wavelength channels.
Many tunable filter technologies have been demonstrated offering various tuning ranges and bandwidths including DFB laser amplifiers, etalons, acousto-optics, and variable-pitch, position-tuned devices based on reflection holograms in dichromated gelatin.
For a device to be suitable for installation in customers' premises, it must employ a technology that lends itself to low cost mass production.
SUMMARY OF THE INVENTION
The present invention provides an optical filter comprising an optical dispersion means, first and second optical waveguides mounted on a common support such that a multiplex of optical wavelength channels launched from one of the optical waveguides into the optical dispersion means is receivable by the other optical waveguide after dispersion by the optical dispersion means, and means for moving the support relative to the dispersion means, whereby said other optical waveguide sweeps through the dispersed wavelength spectrum of the multiplex of wavelength channels, thereby allowing any desired wavelength channel to be coupled into said other waveguide.
The waveguides optically coupled by the dispersion means constitute input and output waveguides of the filter.
Such a tunable filter can use mass produced supports to move the optical waveguides precise, small distances such as are used in compact disc players to control the reading lasers' positions for example, and appears to offer, at present, the best overall performance in terms of insertion loss, optical bandwidth and interchannel crosstalk.
In order to provide two-way broadband services employing high-density wavelength multiplexing in the upstream direction, every customer's laser on a passive optical network must transmit at a distinct wavelength different from all others, and these wavelengths must be accurately positioned to form a high-density multiplex as the customers' signals combine through the optical splitters.
The optical filter may include a third optical waveguide mounted on the common support.
According to a further aspect of the present invention an optical
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Blyth Kevin J.
Hill Alan M.
Payne David B.
British Telecommunications public limited company
Gonzalez Frank
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