the invention relates to an external optical modulator comprising a mach - zehnder having a signal electrode including at least four sections of unequal length to one another positioned over an alternating domain structure in an electrooptic substrate , and including a center section , or center pair of sections disposed asymmetrically between pairs surrounding sections . the surrounding pairs , comprise the two sections adjacent the center section or pair of sections , and each two sections adjacent the previous pair of sections , moving outwardly from the center to the final outermost pair , l 1 and l n at the rf input 2 and rf output 4 . in each pair , the section lengths are equal , or the section closer to the rf output 4 has a longer length than the section closer to the rf input 2 . the surrounding pairs have lengths that decrease from the innermost pair to the outermost pair . for a zero chirp structure , the section lengths are selected to maintain an equivalent length for the inverted and uninverted domain sections . the present invention has found that constant chirp solutions can be found for all frequencies .