Optical signal equalizer for wavelength division multiplexed optical fiber systems

Abstract

This invention disclosure describes the application of a polarization insensitive acoustically-tuned optical filter used in a multichannel WDM system to equalize variations in the power level of the WDM channels. The invention also describes a simple means for providing a low frequency control system which enables the equalizer to determine the signal levels of N optical carriers prior to equalizing the signals.

What is claimed is:  
     
       1. Apparatus in a communication system for controlling the level of a multiplexed optical signal composed of a plurality of amplified optical signals Si  S i  where i is a number denoting one of the plurality of optical signals at an individual frequency fiwavelength λ i  comprising: 
       tap means for tapping said multiplexed optical signal to produce a tapped multiplexed optical signal;  
       means for converting said tapped multiplexed optical signal to demultiplexed electrical signals;  
       control means having an input port for receiving said demultiplexed electrical signals, including a calculation means for calculating coefficients Xi at frequency fi  X i  corresponding to individual wavelengths λ i ; 
       filter means having an input port for receiving said input multiplexed optical signal, a first output port for transmitting a first output optical signal, and including a control port means for receiving said control signal;  
       wherein the filter means responsive to the control signal attenuates the level of one or more of the individual optical signals S  S i  at the first output port according to the relation cos 2 (Xiπ/2)  cos 2 ( X   i   π/2).    
     
     
       2. The apparatus of  claim 1  wherein the filter means is a polarization independent acoustically tuned optical filter. 
     
     
       3. The apparatus of  claim 1  wherein the control signal of the control port means in a multiplexed signal composed of one or more of the  frequencies corresponding to the wavelengths of the individual optical signals. 
     
     
       4. The apparatus of  claim 1  wherein the control signal of the control port means is an electrical signal. 
     
     
       5. The apparatus of  claim 1  wherein the control port means for receiving a control signal is an optical signal. 
     
     
       6. The apparatus of  claim 1  further comprising: 
       a second output port for transmitting a second output signal;  
       wherein the filter means distributes the input optical signal between the first output port and the second output port.  
     
     
       7. The apparatus of  claim 6  wherein the filter means distributes the input optical signal composed of a plurality of optical signals at separate wavelengths at the second output port proportional to sin 2 (Xiπ/2)  sin 2 ( X   i   π/2).