Optical: systems and elements – Light interference – Produced by coating or lamina
Reexamination Certificate
2000-02-14
2001-09-18
Henry, Jon (Department: 2874)
Optical: systems and elements
Light interference
Produced by coating or lamina
C359S589000, C359S578000
Reexamination Certificate
active
06292299
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to optical filters, and more particularly to wavelength-tunable optical filters and their use within WDM add/drop devices.
BACKGROUND OF THE INVENTION
Optical fiber is becoming the transmission medium of choice for many communication networks because of the speed and bandwidth advantages associated with optical transmission. In addition, wavelength division multiplexing (WDM) is being used to meet the increasing demands for higher data rates and more bandwidth in optical transmission applications.
In its simplest form, WDM is a technique whereby parallel data streams of modulated light at different wavelengths are coupled simultaneously into the same optical fiber. As such, a WDM signal is typically viewed as a composite optical signal comprised of a plurality of optical wavelength channels sharing a single transmission medium, each wavelength channel having a different wavelength of light. Although each wavelength channel actually includes a range of wavelengths, it is common to refer to an optical wavelength channel in terms of its center wavelength.
It is often necessary to add or remove a particular wavelength channel at various points along an optical fiber transmission path, without disturbing or disrupting the remaining wavelength channels, whether the optical transmission system is a long haul, metropolitan, or local. Adding or removing a wavelength channel is accomplished utilizing add/drop devices. An add/drop device typically utilizes a bandpass filter, that is, an optical filter that is transmissive with respect to one or more wavelength channels and reflective with respect to the remaining wavelength channels, to add or remove the desired wavelength channel. The remainder of the wavelength channels not within the passband of the filter, remain unaffected by the device, and the transmission of their respective modulated light data streams is unimpeded.
In recent years, tunable filters have been developed which, when incorporated as the optical filter in an add/drop device, enable the device to be dynamically tuned to add or remove a desired optical wavelength channel from the plurality of wavelength channels. If desired to change the added or dropped wavelength channel, it is easily accomplished without having to replace the filter element (or the entire add/drop device) with another having the desired bandpass characteristics. This is typically accomplished by repositioning the filter with respect to an incident optical beam. However, one unfortunate aspect resulting from dynamically tuning an add/drop device is that intermediate wavelength channels, those channels having wavelengths existing between that of the initially tuned channel and that of the finally tuned channel, will each sequentially exhibit a temporary loss of signal continuity as the filter is tuned to each respective channel's wavelength. This occurs because the point of incidence of the optical beam upon the filter, in transitioning from a point corresponding to the initially tuned channel to a point corresponding to the finally tuned channel, passes filter locations corresponding to each of the intermediate channels. When the composite optical signal light strikes filter locations corresponding to intermediate wavelength channels, intermittent data loss from those intermediate wavelength channels results. Such an intermittent loss of data is often referred to as a so-called “hit.” The deleterious effects of a data hit, to the devices for which the data is intended to be transmitted, are well known. Devices exposed to such a data loss must either compensate for the loss of data, or request retransmission of the lost data. Ultimately, such data loss results in diminished quality of service, decreased bandwidth efficiency, or both.
SUMMARY OF THE INVENTION
Data loss is reduced and bandwidth efficiency improved while tuning an optical add/drop device comprising a hitless wavelength-tunable optical filter. The hitless wavelength-tunable optical filter includes a broadband reflective region and a tunable filter region. Hitless tuning of the device is accomplished by changing the point at which an optical beam is incident upon the filter region along a constant wavelength channel track whenever the beam is strikes the device in the filter region. Realignment to a position associated with a new wavelength channel track is performed when the optical beam is incident upon the broadband reflective region. Repositioning the optical beam to the filter region occurs at a location corresponding to the new wavelength to be added/dropped and subsequent optical beam realignment within the filter region is along the new wavelength channel track.
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Eriksen Guy H.
Henry Jon
Lucent Technologies - Inc.
Murgia Gregory J.
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