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.

What is claimed is: 
     
         1 . A tunable optical filter comprising:
 an input port for receiving an optical signal, the optical signal including a plurality of optical frequency channels, each optical frequency channel having a central frequency substantially centered at a different frequency of predetermined frequency grid having a predetermined grid spacing;   an output port for transmitting an optical frequency channel selected from the plurality of optical frequency channels;   a plurality of sequentially coupled tunable Mach-Zehnder (MZ) interferometers optically disposed between the input port and the output port for isolating the selected optical frequency channel from the plurality of optical frequency channels, each tunable MZ interferometer having a plurality of equidistantly spaced conterminous frequency passbands and frequency stopbands and having a free spectral range substantially equal to an integer multiple of the predetermined grid spacing;   a first MZ interferometer optically disposed between the input port and the output port, the first MZ interferometer including first and second interferometer arms optically disposed between first and second optical couplers, the first optical coupler for directing more than 75% of the light received at an input of the first MZ interferometer into the first interferometer arm, the first and second interferometer arms having different lengths, and   a controller for tuning the plurality of sequentially coupled MZ interferometers to have one passband of each MZ interferometer centered on the central frequency of the selected optical frequency channel, and to have at least one of the stopbands of the MZ interferometers centered on the central frequency of each remaining optical frequency channel of the optical signal, so as to suppress each said remaining optical frequency channel of the optical signal, and for tuning the first MZ interferometer to have low transmission at the center frequency of the selected optical frequency channel.   
     
     
         2 . A tunable optical filter according to  claim 1 , wherein the first MZ interferometer has a free spectral range close to the predetermined grid spacing. 
     
     
         3 . A tunable optical filter according to  claim 2 , wherein each of the first and second optical couplers is an asymmetric coupler having a coupling ratio between 75%/25% and 99%/1%. 
     
     
         4 . A tunable optical filter according to  claim 2 , wherein each of the first and second optical couplers is a Mach-Zehnder variable coupler for providing a coupling ratio between 75%/25% and 100%/0%. 
     
     
         5 . A tunable optical filter according to  claim 2 , wherein a transmission spectrum of the first MZ interferometer includes a plurality of equidistantly spaced optical intensity maxima and minima, and wherein the controller is for tuning the first MZ interferometer to have an optical intensity minimum at the center frequency of the selected optical frequency channel. 
     
     
         6 . A tunable optical filter according to  claim 3 , wherein a transmission spectrum of the first MZ interferometer includes a plurality of equidistantly spaced optical intensity maxima and minima, and wherein the controller is for tuning the first MZ interferometer to have an optical intensity minimum at the center frequency of the selected optical frequency channel. 
     
     
         7 . A tunable optical filter according to  claim 4 , wherein a transmission spectrum of the first MZ interferometer includes a plurality of equidistantly spaced optical intensity maxima and minima, and wherein the controller is for tuning the first MZ interferometer to have an optical intensity minimum at the center frequency of the selected optical frequency channel. 
     
     
         8 . A tunable optical filter according to  claim 2 , comprising a second MZ interferometer optically disposed between the input port and the output port, the second MZ interferometer including third and fourth interferometer arms optically disposed between third and fourth optical couplers, the third optical coupler for directing more than 75% of the light received at an input of the second MZ interferometer into the third interferometer arm, the third and fourth interferometer arms having different lengths,
 wherein a transmission spectrum of the second MZ interferometer includes a plurality of equidistantly spaced optical intensity maxima and minima, and   wherein the controller is for tuning the first MZ interferometer to have an optical intensity minimum at a first frequency and the second MZ interferometer to have an optical intensity at a second frequency, the first frequency different from the second frequency, each of the first and second frequencies offset from the center frequency of the selected optical frequency channel by a same magnitude.   
     
     
         9 . A tunable optical filter according to  claim 1 , wherein the plurality of sequentially coupled tunable MZ interferometers produce a substantially Gaussian shaped transmission spectrum, and wherein a transmission spectrum of the first MZ interferometer includes a plurality of equidistantly spaced optical intensity maxima alternating with optical intensity minima, and wherein the controller is for tuning the first MZ interferometer to have at least one optical intensity minimum centered at the center frequency of the selected optical frequency channel. 
     
     
         10 . A tunable optical filter according to  claim 1 , wherein the frequency grid is the ITU frequency grid, and wherein the predetermined grid spacing is one of 50 GHz and 100 GHz. 
     
     
         11 . A tunable optical filter according to  claim 10 , wherein the plurality of the MZ interferometers includes at least 9 stages of MZ interferometers including MZ interferometers having an FSR of 100 GHz, 200 GHz, 400 GHz, 800 GHz, and 1600 GHz. 
     
     
         12 . A tunable optical filter according to  claim 11 , wherein the first MZ interferometer has an FSR that is about equal to the predetermined grid spacing. 
     
     
         13 . A tunable optical filter according to  claim 1 , wherein the first MZ interferometer has an FSR that is between about 50% and 150% of the predetermined grid spacing. 
     
     
         14 . A tunable optical filter according to  claim 8 , wherein each of the first and second MZ interferometers has an FSR substantially equal to the predetermined grid spacing. 
     
     
         15 . A tunable optical filter according to  claim 1 , comprising an optical shutter optically disposed between the input port and the output port for suppressing the optical signal during tuning of said plurality of sequentially coupled MZ interferometers. 
     
     
         16 . A tunable optical filter according to  claim 1 , wherein the plurality of sequentially coupled MZ interferometers and the first MZ interferometer are integrated on a same planar lightwave circuit (PLC) chip. 
     
     
         17 . A tunable optical filter according to  claim 16 , comprising a plurality of local heaters disposed on a top surface of the PLC chip and coupled to the controller, for thermally tuning the plurality of sequentially coupled MZ interferometers and the first MZ interferometer. 
     
     
         18 . A tunable optical filter according to  claim 3 , wherein each of the first and second optical couplers is an asymmetric coupler having a coupling ratio of about 80%/20%. 
     
     
         19 . A method of filtering an optical signal comprising:
 passing an optical signal through a tunable optical filter, the optical signal including a plurality of optical frequency channels, each optical frequency channel having a central frequency substantially centered at a different frequency of predetermined frequency grid having a predetermined grid spacing, the tunable optical filter including:
 a plurality of sequentially coupled tunable Mach-Zehnder (MZ) interferometers for selecting an optical frequency channel from the plurality of optical frequency channels, each tunable MZ interferometer having a plurality of equidistantly spaced conterminous frequency passbands and frequency stopbands and having a free spectral range substantially equal to an integer multiple of the predetermined grid spacing; 
 a first MZ interferometer optically coupled to the plurality of sequentially coupled tunable MZ interferometers, the first MZ interferometer including first and second interferometer arms optically disposed between first and second optical couplers, the first optical coupler for directing more than 75% of the light received at an input of the first MZ interferometer into the first interferometer arm, the first and second interferometer arms having different lengths; and 
 a controller; and 
   tuning the plurality of sequentially coupled MZ interferometers to have one passband of each MZ interferometer centered on the central frequency of the selected optical frequency channel, and to have at least one of the stopbands of the MZ interferometers centered on the central frequency of each remaining optical frequency channel of the optical signal, so as to suppress each said remaining optical frequency channel of the optical signal; and   tuning the first MZ interferometer to have low transmission at the center frequency of the selected optical frequency channel.   
     
     
         20 . A method according to  claim 19 , wherein tuning the first MZ interferometer to have low transmission at the center frequency of the selected optical frequency channel comprises tuning the first MZ interferometer to have an optical intensity minimum at the center frequency of the selected optical frequency channel.