Contents I. Introduction
Phase modulated coherent fiber-optic links have the potential for greatlyenhanced spur free dynamic range (SFDR) [1]. The backbone of a coherent link is the coherent optical receiver used to demodulate the phase of the received signal. A coherent receiver consists of a coupler for mixing the incoming signal with a local oscillator (LO) signal, and a balanced photodiode (BPD) for detecting the phase difference between the two waves. BPDs are employed because of their ability to suppress laser relative intensity noise (RIN) [2]. Some of the desired attributes for a coherent receiver are high power, high linearity, and large CMRR. A pin BPD has been reported with more than 20 dB CMRR [3]. UTC-PDs are attractive for BPDs because of their high saturation current capabilities made possible by reduced space charge effects [4]. Recently an advanced device architecture has been developed allowing for the monolithic integration of waveguide UTC-PDs with other optical components [5]. A monolithically integrated coherent receiver is advantageous because the optical power coupled to the PDs can be maximized. In this work, we have fabricated and tested a monolithically integrated BPD consisting of two series connected UTC-PDs, a tunable 2⨯2 MMI coupler, and DC decoupling capacitors. With a single regrowth process, several optical components have been monolithically integrated to form a highly functional microwave photonic circuit. The UTC-PDs exhibit both high saturation current and high linearity. The MMI can be fine tuned to a 50:50 splitting ratio allowing for accurate control of the power balance in the PDs. The BPD is also highly symmetric lending to a large CMRR. (a) SEM image of UTC-PD pair connected in series and (b) schematic of balanced UTC-PD with integrated tunable MMI coupler.
