An optical filter

Abstract

The invention relates to an optical filter. An apparatus and method are provided for filtering an optical signal by wavelength. An initially polarised signal is passed through a DGD element effective to alter the polarisation state of the components of the signal according to wavelength. A polarisation filter (polariser) is then provided to attenuate light having given polarisation states from the signal, thereby attenuating the wavelengths associated with said polarisation states. The invention finds particularly utility in the domain of vestigial sideband filtering, allowing the bandwidth of signals within a wavelength division multiplexed system to be reduced without introducing the deleterious effects associated with conventional wavelength filters.

Description

technical field [0001] The invention relates to filtering of optical signals. More particularly, it relates to the filtering of optical signals in long-distance communication systems for higher spectral efficiency. Background technique [0002] Wavelength division multiplexing (WDM) signals are commonly used in optical transmission systems. WDM signals combine multiple source signals (channels) at different frequencies to maximize the amount of data that can be transmitted simultaneously over a given fiber. However, since any data signal will have a finite bandwidth, there is a finite number of channels that can be satisfactorily transmitted within a given frequency range, and this limitation in the number of channels occurs when the frequency spectrum of adjacent channels begins to overlap . Spectral overlap can degrade transmission performance due to crosstalk between adjacent signals. Moreover, the greater the width of the signal, the greater the distortion effects of...

AI Technical Summary

Problems solved by technology

[0006] Another disadvantage of conventional VSB filtering techniques is that, once the filter is manufactured, it is often difficult to control the wavelength at which the optical filter acts

Because of this, WDM systems using VSB filtering are generally inflexible and cannot adapt to new circumstances when channel frequencies need to be changed (for example, a change in the number of channels would cause the system to readjust to best use the available bandwidth)

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