US 6,724,786 B2Granted

Variable optical attenuator using wavelength locked loop tuning

AssigneeInternational Business Machines Corporation

InventorsLawrence Jacobowitz, Casimer Maurice DeCusatis

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Abstract

A system and method for automatically attenuating optical signals transmitted in optical systems. The system and method exploits a wavelength-locked loop servo-control circuit and methodology that enables real time mutual alignment of the center wavelength of an optical signal having a peaked spectrum function and transmitted through the optical system, and a center wavelength of a wavelength selective device such as an optical filter element implementing a peaked passband function. The wavelength-locked loop servo-control circuit and methodology particularly is capable of real-time aligning the center wavelength of an optical signal in a range between maximum overlap with the center wavelength of the peaked passband function of the optical filter for maximum transfer of output optical signal by the filter element and minimum overlap with the peaked passband function of the optical filter so that output optical signal may be attenuated in the optical system.

Claims16

Dependent claims

An apparatus for attenuating optical signals communicated in an optical network comprising:

optical signal generator for generating optical signals, each optical signal having a peaked spectrum function including a center wavelength;
an optical filter element for receiving and filtering an optical signal, said optical filter element exhibiting a peaked passband function including a center wavelength;
and,
a wavelength-locked loop servo-control circuit for enabling real time alignment of said optical signal center wavelength with said peaked passband function of said optical filter element, said optical signal center wavelength capable of being aligned at a wavelength corresponding to maximum overlap with said center wavelength of said peaked passband function of said optical filter for maximum transfer of said output optical signal by said filter element and minimum overlap with said peaked passband function of said optical filter so that said output optical signal may be attenuated in said optical system, said wavelength-locked loop servo-control circuit comprising:
a means for applying a dither modulation signal at a dither modulation frequency to said optical signal to generate a dither modulated optical signal through said optical filter element;
a means for converting a portion of dither modulated optical signal to into a feedback signal;
a means for generating an error signal comprising a vector cross product of said feedback signal and said dither modulated optical signal;
and
a means responsive to said error signal for adjusting the peak spectrum function of said optical signal according to a desired amount of optical signal attenuation, wherein said center wavelength of said optical signal is adjustable between said maximum overlap and minimum overlap with said center wavelength of said peaked passband function of said optical filter and becomes aligned for maximum overlap with said center wavelength of said peaked passband function of said optical filter when said frequency characteristic of said feedback error signal is two times said dither modulation frequency.

6 more elements

A method for attenuating optical signals communicated in an optical network comprising the steps of:

a) providing optical signal generator for generating optical signals, each optical signal having a peaked spectrum function including a center wavelength;
b) providing optical filter element for receiving and filtering an optical signal, said optical filter element exhibiting a peaked passband function including a center wavelength;
and,
c) enabling real time alignment of said optical signal center wavelength with said peaked passband function of said optical filter, said optical signal center wavelength capable of being aligned at a wavelength corresponding to maximum overlap with said center wavelength of said peaked passband function of said optical filter for maximum transfer of said output optical signal by said filter element and minimum overlap with said peaked passband function of said optical filter so that said output optical signal may be attenuated in said optical system, said real-time alignment step further comprising:
d) applying a dither modulation signal at a dither modulation frequency to said optical signal to generate a dither modulated optical signal through said optical filter element having a peak frequency response at a desired wavelength;
e) converting a portion of dither modulated optical signal to into a feedback signal;
f) generating an error signal comprising a vector cross product of said feedback signal and said dither modulated optical signal;
and
g) adjusting the peak spectrum function of said optical signal according to a desired amount of optical signal attenuation, wherein said center wavelength of said optical signal is adjustable between said maximum overlap and minimum overlap with said center wavelength of said peaked passband function of said optical filter and becomes aligned for maximum overlap with said center wavelength of said peaked passband function of said optical filter when said frequency characteristic of said error feedback signal is two times said dither modulation frequency.

6 more elements

An apparatus for providing gain equalization in a wavelength division multiplexing system comprising an array of optical signal generators each for generating an optical signal having a peaked spectrum function including a center wavelength, and a corresponding array of optical filter elements, each filter element for receiving and filtering a corresponding optical signal from said optical signal generator array, each said optical filter element exhibiting a peaked passband function including a center wavelength, said apparatus comprising:

a wavelength-locked loop servo-control circuit for enabling real time alignment of each said optical signal center wavelength generated by said optical signal generator array with said peaked passband function of its respective optical filter element in said filter array, each said optical signal center wavelength capable of being aligned at a wavelength corresponding to maximum overlap with said center wavelength of the peaked passband function of its respective optical filter for maximum transfer of said output optical signal by its respective filter element and capable of being aligned at a wavelength corresponding to minimum overlap with said peaked passband function of its respective optical filter so that said output optical signal may be attenuated in said optical system, said wavelength-locked loop servo-control circuit comprising:
a means for applying a dither modulation signal at a dither modulation frequency to each said optical signal of said array to generate a plurality of dither modulated optical signals for transmission through its respective optical filter element;
a means for converting a portion of each dither modulated optical signal into a corresponding feedback signal;
a means for generating a respective error signal comprising a vector cross product of said feedback signal and said dither modulated optical signal;
and
a means responsive to each respective error signal for adjusting the peak spectrum function of its corresponding optical signal according to a desired amount of optical signal attenuation, wherein a center wavelength of each said optical signal is adjustable between said maximum overlap and minimum overlap with said center wavelength of said peaked passband function of its respective optical filter and may become aligned for maximum overlap with said center wavelength of said peaked passband function of said respective optical filter when said frequency characteristic of said feedback error signal is two times said dither modulation frequency.

3 more elements

Dependencies are inferred from the claim text.