Novel design for the broadband linearized optical intensity modulator

Abstract

The field of microwave photonics in which the radio-frequency signal is transmitted using an optical link has been experiencing rapid growth in the last decade. There is a demand for compact, robust and inexpensive devices capable of achieving high fidelity analog transmission between two remote locations. Analog transmission possesses an inherent advantage of being transparent to various modulation formats in many bands. Many applications require large dynamic range (over 130 dB/Hz) and large (few GHz) bandwidth. In such applications the element limiting the performance turns out to be the optical intensity modulator. The typical intensity modulator based on the Mach-Zehnder interferometer (MZI) has a nonlinear response and thus exhibits strong third-order intermodulation distortion limiting the dynamic range. Numerous suggestions for linearization of the MZI response have not become practical due to their complexity of design. We propose a novel, exceptionally simple way of linearization of the intensity modulator response based on combining a MZI and a low-Q off-resonant Gires-Tornois interferometer (GTI) in one integrated package. In this design the third-order intermodulation distortion can be completely cancelled raising the spur-free dynamic range to 130 dB in 1 Hz bandwidth. At the same time, due to the low Q of the GTI a bandwidth in excess of 3 GHz can be attained anywhere in a 0-45 GHz range.