IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 5, MARCH 1, 2011 311 InP/InGaAsP-Based Integrated 3-dB Trench Couplers for Ultra-Compact Coherent Receivers Uppiliappan Krishnamachari, Sasa Ristic, Chin-Hui Chen, Leif Johansson, Anand Ramaswamy, Jonathan Klamkin, Erik Norberg, John E. Bowers, Fellow, IEEE, and Larry A. Coldren, Fellow, IEEE Abstract—We present the design, fabrication, and test results for ultra-compact 3-dB frustrated total internal reflection-based trench couplers in an InP/InGaAsP monolithic integration plat- form. The trench coupler is integrated with phase modulators and a balanced photodiode (BPD) pair to enable a 180 -hybrid ultra- compact coherent receiver. Several trench splitter designs exhibit near 3-dB splitting with a loss of 3 dB. The BPD pair is used to characterize coherent mixing of two input optical signals into the trench splitter, and coherence efficiency of 75% is achieved. Index Terms—Beam splitter, coherent mixing, etched slot, frus- trated total internal reflection (FTIR), 3-dB coupler, trench. I. INTRODUCTION W AVEGUIDE couplers are important components in the realization of compact, integrated optical circuits due to their ability to split the light beam or change its direction in a short distance [1]. A compact optical mixing element is re- quired to minimize the footprint and optical path length. The most commonly used beam splitter for photonic integrated cir- cuits is the multimode interference (MMI) coupler, and although recent advances in MMI design have yielded lengths as short as 50 m, the devices are still limited in geometry due to ra- diation loss suffered in sharp bends [2]. In contrast, an etched trench that cuts the optical waveguide can perform 3-dB split- ting within a submicrometer length. This is achieved by using the trench as a frustrated total internal reflection (FTIR) mirror, where the angle of the input waveguide incident on the trench is greater than the critical angle [3]. In a process analogous to quantum mechanical tunneling, the incident wave creates an evanescent field that penetrates into the lower index medium of the trench. If the gap width is small enough, this evanes- cent wave can couple across the gap to the waveguide on the other side and form the transmitted wave. The reflected wave still behaves as a totally internally reflected wave, exhibiting a small lateral Goos–Hanchen shift from the incident wave. The reflected and transmitted waves are complementary, behaving Manuscript received August 30, 2010; revised October 27, 2010; accepted November 27, 2010. Date of publication December 17, 2010; date of current version February 24, 2011. The authors are with the University of California, Santa Barbara En- gineering Science Building, Electrical and Computer Engineering, Santa Barbara, CA 93106-9560 USA (e-mail: ukrishna@ece.ucsb.edu; ristic@ece. ucsb.edu; janet@ece.ucsb.edu; leif@ece.ucsb.edu; anand@ece.ucsb.edu; klamkin@engineering.ucsb.edu; norberg@ece.ucsb.edu; bowers@ece.ucsb. edu; coldren@ece.ucsb.edu). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2010.2100376 Fig. 1. SEM image of trench coupler illustrating reflection and transmission of a wave incident on the trench coupler. Fig. 2. Schematic of trench splitter-based coherent receiver. as a 180 hybrid [4]. The splitting of the incident wave into re- flected and transmitted signals is shown in Fig. 1. The theoretical framework for an FTIR based 3-dB coupler design has been explained elsewhere [5]. Demonstration of FTIR trench beam splitters has been reported in both Al- GaAs and InGaAsP material systems [6], [7]. In this letter we present a coherent receiver structure with a trench beam splitter integrated with phase modulators and a balanced uni-travel- ling-carrier photodiode (UTC-PD) for use in an ultra-compact coherent optical receiver with feedback [8]. A schematic of the coherent receiver can be seen in Fig. 2. II. DESIGN AND SIMULATION In this design, we use benzocyclobutene (BCB) with a re- fractive index of 1.57 to fill the trench. With a calculated ef- fective index of 3.265 for the InGaAsP optical waveguide, the critical angle for the semiconductor/BCB interface is 28.5 . We fabricated waveguides with values of crossing angle, , that range from 27 –32 to account for any error in the index of the semiconductor or the BCB. Two-dimensional finite-differ- ence time-domain (FDTD) simulations were carried out for each of these angles in order to find the gap width corresponding to 1041-1135/$26.00 © 2010 IEEE