Highly Linear InP Phase Modulator for High Dynamic Range RF/Photonic Links Renyuan Wang + , Ashish Bhardwaj # , Sasa Ristic # , Larry Coldren # , John Bowers # , Peter Herczfeld * and Yifei Li + + University of Massachusetts at Dartmouth, MA 02747, USA * Drexel University, Philadelphia, PA 19104, USA # The University of California at Santa Barbara, CA 93106, USA Abstract — The optical phase locked loop (OPLL) photonic integrated circuit (PIC) is a key element for the emerging linear coherent RF-photonic links. One of the main challenges for the OPLL-PIC is the nonlinearity of the Indium Phosphide (InP)- based phase modulator. In this paper, we report the experimental results from a multi-quantum well phase modulator fabricated on an InP substrate that is specially designed for the OPLL-PIC. The phase modulator shows low optical loss and good linearity performance. In particular, at a reverse bias voltage of 5.6 V, its phase IP3 and insertion loss per unit length are ~2.8π/mm and 1.2 dB/mm, respectively. Index Terms —optical phase locked loop, dynamic range, quantum wells, linear phase modulator. I. INTRODUCTION nalog fiber-optic links are attractive for Radar front-end applications in which they connect the antennas to a signal processing unit. The conventional IM-DD fiber-optic links have inadequate spurious free dynamic range (SFDR) due to their intrinsic nonlinearity [1-2]. A novel coherent phase modulated (PM) optical link employing an attenuation- counter-propagating (ACP) optical phase locked loop (OPLL) linear phase demodulator has been proposed for solving the problem arising from the limited dynamic range [3-4]. As shown in Fig. 1, the ACP-OPLL contains an ACP-optical phase modulator, a 3 dB coupler and a pair of balanced high power photodiodes. Using feedback, the ACP-OPLL forces the photodiode output to be a scaled replica of the RF signal driving the phase modulator at the transmitter side. This new optical link aims to achieve a spurious free dynamic range (SFDR) nearing 140 dB·Hz 2/3 , significantly surpassing the present state-of-the-art. In order to implement a high bandwidth ACP-OPLL, the ACP-OPLL must be implemented as compact photonic integrated circuit (PIC), where its loop propagation delay is kept at a minimum. An appropriate choice for implementing the local phase modulator inside the ACP-OPLL PIC is an In x Ga 1-x As y P 1-y -based multi-quantum well (MQW) phase modulator that can be monolithically integrated with low-loss waveguides and photodetectors on an Indium Phosphide (InP)-based material platform. The major disadvantage with using InP phase modulators is their nonlinearity. The linear modulation range of the phase modulator is characterized by its phase IP3 [6,7], which is defined as the intercept point of the linear phase modulation response and the third order spurious response in its output phase. Although in theory the phase IP3 of the phase modulator scales linearly with the device length, the high optical loss of existing InP-based optical phase modulators sets a practical constraint on the modulator length and consequently limits the phase IP3 in the range of a couple of  . However, to achieve an SFDR over 140 dB·Hz 2/3 , a phase IP3 of 10π or larger is required for the phase modulator. In this paper, we present a novel InP-based MQW phase modulator design with a deep ridge optical waveguide which shows a very low optical loss and a good phase IP3 value. This MQW phase modulator design enables an ACP local phase modulator with a phase IP3 of 10 and a 4.3 dB optical loss. In the following sections, we first present the design of the InP-based MQW phase modulator, and then report experimental results from the phase modulator. ACP-PPLL Phase Modulator RF input Fiber Link Laser V out V in =A(t)exp(i m t)+c.c Z L ACP in-loop phase modulator BPD Laser TX S TX (t) S LO (t) Z ACP Fig. 1: PM fiber-optic link with ACP-OPLL phase demodulator/detector. II. INP MQW PHASE MODULATOR DESIGN InP-based MQW optical phase modulator devices are inherently nonlinear. In order to enhance its linear phase modulation range, the most straightforward approach is to increase the modulator length. This, however, increases the optical propagation loss through the device. For mitigation, we selected a quantum well design [5] where the operating wavelength (~1550 nm) is far from the quantum well absorption peak. A schematic of the device cross-section is shown in Fig. 2 along with an SEM image of the device. The phase modulator has a p-i-n diode configuration and is fabricated on a semi-insulating InP substrate. The p-doped layers of the device above the intrinsic MQW region consists of a highly doped (2×10 19 cm -3 ) p-InGaAs contact layer and three subsequent p-InP layers with doping levels of 1×10 18 cm - A