Patent ID: 6856751

Claim:
A method of operating a variable optical attenuator to provide a required total attenuation in a planar lightwave circuit, comprising: a) providing a first Mach-Zehnder interferometer having a first and a second arm carrying signals in an arbitrary polarization state therethrough, b) providing a second Mach-Zehnder interferometer having a third and a fourth arm carrying signals in an arbitrary polarization state therethrough, said second Mach-Zehnder interferometer concatenated in tandem with said first Mach-Zehnder interferometer, and c) cooperatively operating said concatenated first and second Mach-Zehnder interferometers to obtain the required total attenuation and an effectively nil polarization dependent loss by: i. operating said first Mach-Zehnder interferometer to provide both a total first attenuation and, for each said signal in an arbitrary polarization state, a first polarization dependent loss having a first sign, and ii. operating said second Mach-Zehnder interferometer to provide both a total second attenuation and, for each said signal in an arbitrary polarization state, a second polarization dependent loss having a second sign opposite to said first sign, whereby the sum of said first and said second total attenuations equals the total required attenuation, and whereby said first and said second polarization dependent losses effectively cancel each other.