Optical communications system including doped optical fiber filt

Details Optical communications system including doped optical fiber filt Optical communications system including doped optical fiber filt

1993-08-25

1997-01-14

Pascal, Leslie

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

359114, 359173, 385 24, H04J 1402, H04B 1012

Patent

active

055945783

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to optical communications systems and in particular to systems in which a plurality of wavelengths are used to provide distinct communications channels over single optical fibres.
As the use of optical fibres becomes more widespread there is an increasing desire and need to use more of the potential bandwidth of the fibres. A favoured way of improving the use of the available bandwidth is to use multiple wavelengths on a single fibre, the different wavelengths each providing a different distinct communications channel. This is usually called "Wavelength Division Multiplex" or "WDM".
The increasing use of WDM optical fibre systems means that there is likely to be an increasing need to separate the wavelengths each into its own different fibre. This separation can be achieved by a wavelength sensitive splitter often known as a "demodulator" or "DMX". It should be noted that systems will also comprise splitters which are not wavelength sensitive in order to permit two-way communication along the same fibre.
British Telecom's passive optical network (PON) [described in British Telecom Technology Journal, 1989, Volume 7, pp 89-99] is an example of an optical fibre communications system which utilises optical filters. The PON approach aims to reduce the cost of providing small businesses and residential users with direct access to an optical fibre network by using the fibre bandwidth to share one fibre amongst a plurality of customers, reducing the amount of plant in the ground and also sharing the exchange equipment. The `passive` element comes from the desire to eliminate the need for street-mounted electronics, hopefully restricting active elements to the exchange and customers' premises. A time division multiplexed (TDM) signal is broadcast to all terminals from the exchange on a single wavelength i.e. the telephony wavelength, with the customer time accessing the particular bits meant for him. In the return direction, data from the customer is inserted at a pre-determined time to arrive at the exchange in synchronism with other customers' data. Inclusion of an optical filter in the customer's terminal that passes only the telephony wavelength allows the later provision of new services on other wavelengths without disturbing the telephony transmission. In a field trial which is currently under way, a single exchange fibre has a 128 way split operating at 20 Mbit/s to provide ISDN services to all customers.
As mentioned above the TDMX system uses a single wavelength and the capacity of the system can be further increased, e.g. to incorporate cable TV, by using a WDM. Depending upon the system requirements, each wavelength channel of the WDM may also operate as TDMX. Such systems are based on the concept of providing a separate wavelength range for each of telephony (TPON), broadband services (BPON), and maintenance. In the aforementioned trial, telephony uses the 1.3 .mu.m window, which ranges between 1.26 and 1.34 .mu.m, since lasers in this wavelength range are cheaper than those in the 1.55 .mu.m window. The 1.55 .mu.m window is divided into four channels at 1.50 .mu.m, 1.525 .mu.m, 1.55 .mu.m and 1.575 .mu.m. Of these the longest is assigned to maintenance whilst the others are available for broadband services. The channels are sufficiently far apart to allow a 15 nm band-pass filter to accept one and reject the others. A 15 nm filter bandwidth is compatible with DFB lasers. The 25 nm spacing between the channels is consistent with that available from commercial gratings WDMs which could be used to separate broadband channels.
It has been proposed to provide each telephony customer with a filter which passes only the telephony wavelengths. Currently that filter is a multilayer dielectric interference filter on a thin (100 .mu.m) silicon substrate. The 1 mm square filters are mounted in a precision machined slot cut through a ceramic fibre-connector ferrule, with signal fibres inserted from both ends. The filter is `sandwiched` between the fibre ends, with a typical loss at 1.

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