Feedforward error correction in interferometer modulators

Claim 2

In a interferometer type amplitude modulator incuding: an input hybrid coupler; an output hybrid coupler; and first and second wavepaths, each of which includes a signal phase modulator, for connecting a pair of conjugate branches of said input coupler to a pair of conjugate branches of said output coupler; the improvements comprising: first and second coupling means for extracting a portion of the phase modulated carrier signal derived from each of said phase moduLators; means, utilizing said signal portions, for generating an error signal proportional to the difference between the amplitude of the sum of said signal components and the amplitude of the input modulation signal divided by the cosine of the phase modulation produced by said modulators; and error correcting phase modulation means located along said wavepaths for phase modulating each of said derived signals in response to said error signal in a sense to minimize the amplitude distortion in the output signal from said output coupler.

Claim 3

The modulator according to claim 1 wherein the means for forming said error signal comprises: a 3dB hybrid coupler having the branches of one pair of conjugate branches coupled, respectively, to said first and second coupling means; the other pair of conjugate branches of said 3dB coupler including a third branch wherein a difference signal is formed whose amplitude is proportional to the sine of the phase modulation impressed upon the carrier frequency signals by said signal phase modulators in response to an applied modulation signal e(t), and a fourth branch wherein a sum signal is formed whose amplitude is proportional to the cosine of the phase modulation impressed upon said carrier frequency signals by said modulation signal; a first amplitude detector coupled to said third branch; a second amplitude detector coupled to said fourth branch; means for coupling the output from said first detector to a difference amplifier along with a component of said modulating signal to form an error signal dA; means for dividing said error signal dA by the output from said second detector to form a weighted error signal dA/cos phi (t); and means for coupling said weighted error signal to each of said error connecting phase modulation means.