Broadband Electro-Optic Traveling-Wave Light Modulators

B r o a d b a n d microwave modulation of light requires the use of an extended traveling-wave structure. In order to obtain a c u m u l a t i v e inter2621 2022 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1963 action between the microwave and optical traveling waves, some form of velocity matching is necessary. In a structure of finite length a practical velocity matching criterion is t h a t the component of the optical phase velocity vector taken along the microwave wave vector be equal to the microwave phase velocity. Several possible arrangements immediately suggest themselves for realizing this type of interaction. For example, we could use true traveling-wave structures 1 " 4 or iterative structures 5 where the optical and modulating signal are adjusted to interact intermittently with the proper phase. Broadband modulation further requires t h a t the modulating structure have little dispersion, which implies propagation in a T E M or TEM-like mode. Thus, since the dielectric constants of usefully transparent modulating media are much less at optical frequencies than at microwave frequencies, broadband microwave modulation of light can be achieved by using some form of optical slow-wave structure, of which the simplest is t h a t first proposed by Rigrod and Kaminow. 1 In this structure, shown schematically in Fig. 1, an optical plane wave zigzags back and forth in an electro-optic medium between plane mirrors while the interacting microwave modulating signal propagates longitudinally down the structure.