Flat-top tunable filter

Abstract

A tunable PLC optical filter having sequentially connected thermally tunable Mach-Zehnder (MZ) interferometers is described. The cascade of MZ interferometers, each having a free spectral ranges matching ITU frequency grid spacing, are tuned so as to have a common passband centered on the frequency of the signal being selected, while having at least one of the stopbands centered on any other ITU frequency. Any other optical channel that may be present at any other ITU frequency is suppressed as a result. Another MZ interferometer in series with the cascade of interferometers including an asymmetric or variable coupler, is tuned to have low transmission at the center frequency of the selected optical channel.

Description

TECHNICAL FIELD[0001]The present invention relates to a tunable optical filter, and in particular to a flat-top tunable optical filter having cascaded Mach-Zehnder interferometers.BACKGROUND OF THE INVENTION[0002]Optical filters are often used to select at least one optical frequency band, called a passband, out of an optical frequency spectrum of an optical signal. A central frequency of the passband of a tunable optical filter is adjustable, depending upon a control parameter common to a particular filter type. For example, for a bulk optic tunable filter, the control parameter can be a filter tilt or a clocking (rotation) angle with respect to an incoming optical beam. For an optical waveguide based tunable filter such as tunable Mach-Zehnder (MZ) interferometer, the control parameter can be an electrical signal applied to a localized heater that changes the optical path length of one of its arms, which effectively tunes the MZ interferometer.[0003]Tuning range, spectral selectiv...

AI Technical Summary

Benefits of technology

The present invention provides a tunable optical filter that can isolate and filter optical frequency channels with high accuracy. The filter includes a plurality of sequentially coupled tunable MZ interferometers with different FSRs, which can be tuned to have a flat-top passband shape. The filter has a low modulation and can effectively suppress unwanted optical frequency channels. The filter can be easily tuned to have a desired center frequency and stopband for each optical frequency channel. The technical effects of the invention include improved accuracy and efficiency in optical signal filtering.

Problems solved by technology

Ideally, a tunable filter has excellent crosstalk suppression, since poor crosstalk suppression leads to undesired “leaking” of the optical channels being suppressed, thus impairing the signal level measurements and / or detection and decoding of the selected signal.

Detrimentally, the optical filter of Henry et al. is not tunable.

Unfortunately, the tunable optical filter requires many MZ stages, including stages that have to be repeated, to achieve a satisfactory crosstalk suppression.

Images