Highly linear Y-fed directional coupler modulator with low intermodulation distortion

In this paper, we explore the feasibility of linearizing Y-fed directional couplers. We demonstrate that the linearity of their performance can be enhanced significantly by the appropriate selection of the interaction lengths with respect to the coupling length. The proposed linearization technique brings about the advantage of highly linear performance combined with a simple device design. The single- and two-section embodiments of the Y-fed coupler are compared both in terms of the linearity of the transfer curve and the level of the third-order intermodulation distortion. We show the two-section Y-fed coupler to be the device of choice since it can exhibit high linearity and improved tolerance to fabrication imperfections. We have determined that if the lengths of the two sections are set to 4.1 and 1.05 times the coupling length, respectively, the transfer curve is linear within 1% of the maximum output power over 98% of the entire interval of modulation. Advantageously, a 100% optical modulation depth is achieved at the same time. On the other hand, if these relative lengths are set to 2.3 and 1.05, the third-order intermodulation distortion in the optical output can be significantly suppressed (by more than 100 dB below the carrier). While comparable performance is possible with other linear modulator schemes of more complex designs, practical applications demand simple design with few or no controls. In this regard, the Y-fed modulator is known to benefit from the built-in quadrature point and nearly equal time-averaged power levels in the arms. The former feature can minimize or even eliminate the necessity of using bias voltages, while the latter minimizes potential impact of optical damage on device performance. Furthermore, we also describe a preferred scheme of a two-section Y-fed coupler with a domain reversal in a ferroelectric material. In this case, only a simple uniform electrode structure is utilized which is fully compatible to a single driving source and hence, is convenient for high-speed applications.