Filter Selector

Details Filter Selector Filter Selector

1998-03-18

2001-01-30

Negash, Kinfe-Michael (Department: 2733)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C385S024000

Reexamination Certificate

active

06181451

ABSTRACT:

BACKGROUND OF THE INVENTION
Field of the Invention
In optical telecommunications transmission, there is a growing use of wavelength division multiplexing (WDM). In current WDM systems, different wavelengths of invisible light are each modulated in some way, and combined by multiplexing onto the same optical fibre. The modulation of each wavelength may be analogue or digital, with various rates and protocols, or combinations of them, in order to carry telecommunications traffic such as data, video and voice. At a receiving equipment, conventionally the different wavelengths are separated and demodulated individually.
It is economically attractive to be able to access just some of the wavelengths passing along a fibre, without incurring the expense and other consequences of demultiplexing them all. Access could involve removing or copying light from the fibre at a given wavelength, and/or adding light at the same, or at another wavelength if there is a gap in the spectrum of light already on the fibre. An equipment which provides this feature is a wavelength add-drop multiplexer (WADM). Typically 16 wavelengths might be carried on one fibre, and access to up to say 4 of them might be needed at one site.
It is especially important to be able to change or reconfigure the selection of wavelengths which are to be accessed by remote control. This is in keeping with the general trend of telecommunications transmission towards “managed bandwidth”, by which traffic is increasingly routed around the network as required under central control, via network elements such as SDH (Synchronous Digital Hierarchy) add-drop multiplexers (ADMs) and digital cross-connects.
Various filter technologies exist for WDM multiplexing and demultiplexing, and some are applicable to WDMs. Most of the technologies which allow wavelength access to be configurable have significant performance limitations. This reflects the common fact in systems engineering that an element which is variable in one parameter often performs more marginally in that parameter and also in other parameters, compared with a fixed element.
One way to overcome this problem is to use a set of reflective wavelength filters, each fixed in its wavelength selection, and to introduce different filters into the through path as required. Such filters can achieve high performance in terms of extinguishing the selected wavelength while not affecting others. The introduction of a specific filter would cause its wavelength, when received, to be reflected to one side and thereby dropped to a specified user, and would allow light of the same wavelength to be introduced or added from the side and reflected into the onwards through path.
The potential hazard which arises is that the change from one wavelength filter to another could disturb the total traffic in some way, by temporarily interrupting other wavelengths. Even a disturbance lasting a few nanoseconds or less could cause traffic loss lasting many milliseconds or more, because of the nature of the traffic typically carried. Similarly, a minor but sudden change in the path length of the light, such as by total removal of a filter, could cause disruption. Many telecommunications applications, such as transmission of SDH traffic, cannot tolerate such disruption. The present invention avoids removal of the filter substrate from the path.
One possible way to provide selection between a number of filters is to move each in turn into a light beam, while ensuring by some means that the transition between filters
5
does not disrupt wavelengths for which filters are not being applied. The light beam can be formed by lenses, such that light leaves a fibre, is focussed into a parallel beam of light which passes through space into another lens and is refocussed into another fibre. Suitable commercial devices exist for this purpose, and filters or other components could be introduced into the beam, which may typically be 20 mm long.
SUMMARY OF THE INVENTION
According to the present invention there is provided in an optical telecommunications system using wave division multiplexing (WDM) a wavelength add-drop multiplexer (WADM) comprising an array of filters adjacently connected, each for selecting light of a respective wavelength being transmitted in a multiplexed data stream along an optical fibre, each of the array of filters being adjacently connected to a portion of the array which is a nominally transparent filter and means for moving the array so that movement between filters in use occurs via the nominally transparent filter portion of the array.
There is further provided in an optical telecommunications system using WDM, a method of operating a WADM wherein the transition between two filters of an array of filters each for selecting light of a respective wavelength being transmitted in a multiplexed data stream along an optical fibre occurs by way of a nominally transparent filter portion of the array.


REFERENCES:
patent: 5706375 (1998-01-01), Mihailov
patent: 5726785 (1998-03-01), Chawki et al.
patent: 6020986 (2000-01-01), Ball
patent: 0 733 921 A2 (1996-09-01), None

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