An Ultra-Compact Integrated Coherent Receiver for High Linearity RF Photonic Links Uppiliappan Krishnamachari, Sasa Ristic, Anand Ramaswamy, Leif A. Johansson, Chin-Hui Chen, Jonathan Klamkin, Molly Piels, Ashish Bhardwaj, Mark Rodwell, John E. Bowers, Larry Coldren Electrical and Computer Engineering Department University of California, Santa Barbara Santa Barbara, U.S.A ukrishna@ece.ucsb.edu Abstract—We demonstrate a novel photonic integrated circuit(PIC) that combines an ultra compact trench beam splitter with monolithically integrated detectors and modulators. A coherent receiver is realized by flip chip bonding of this PIC with an electronic integrated circuit(EIC). Preliminary system results yield an IMD3 distortion suppression of 46dB at a signal frequency of 300MHz. I. INTRODUCTION Analog optical links are commonly used in the transmission of signals from antennas and electrical sensors from remote locations. Since these links are designed to convey low level signals from the antenna, high linearity and low-noise operation is required to retain the fidelity of the input signal as closely as possible. The emphasis of this work is on the receiver end of the link which currently poses the greatest challenge in terms of system linearity requirements [1]. Linearity of analog optical receivers is limited in part by higher-order harmonic distortions of the RF signal. Due to system nonlinearities, spurious intermodulation products are created that mask or mimic real signals [2]. The third order intermodulation distortion(IMD3) is of particular concern because it falls within the useful bandwidth of the system. The figure of merit used to quantify the impact of the IMD3 on the system linearity is the spurious-free dynamic range(SFDR), and is defined as the ratio of the largest signal the system can transport to the smallest. Nonlinear distortion limits the maximum signal power and noise limits the minimum detectable signal power, comprising the upper and lower bounds of the dynamic range, respectively [3]. In traditional interferometer-based intensity modulated direct detection(IMDD) photonic links, the dominant limitation on dynamic range is the relative intensity noise, which is proportional to the square of the average optical power [4]. Furthermore, the modulation depth of MZM-based IMDD systems is limited to a range of 0-100%. In contrast, coherent phase modulated(PM) optical links are able to approach shot noise limited performance and the modulation depth can be much greater than 100%. Increasing the optical modulation depth leads to higher signal to noise ratio(SNR) and therefore higher dynamic range. Another limit of overall link dynamic range is that any interferometer based receiver will have a sinusoidal response that will cause inherent nonlinearity, diminishing the benefits mentioned above. To overcome this limitation, we have previously proposed a coherent optical receiver that utilizes a reference tracking modulator with feedback [5]. The receiver architecture, shown in Fig. 1, consists of two phase modulators in push-pull configuration, two photodetectors in balanced configuration, a coupler for mixing the incoming signal with a local oscillator(LO) signal, and an electronic transconductance amplifier circuit. The local oscillator signal is split into two common paths each with a phase modulator. The input signal is applied to the first modulator, recombined with the LO in the coupler, and detected. The balanced PD produces an error signal which is amplified and fed back to the second phase modulator to compensate the phase difference between the signal and local oscillator. This effectively reduces the signal swing of the demodulator thereby restricting the operation of the receiver to the linear region of the response curve. II. DEVICE DESIGN AND FABRICATION In the first iteration of the coherent optical receiver, 2x2 multimode interference (MMI) couplers are used as the mixing element [6]. This material is based upon work supported by the DARPA-PHOR-FRONT program under United States Air Force contract number FA8750-05-C-0265