Tolerant, broadband tunable 2 × 2 coupler circuit

Mi Wang,Wim Bogaerts,Yufei Xing,Antonio Ribero

doi:10.1364/oe.384018

Abstract

We propose a circuit design for a broadband tunable 2 × 2 waveguide coupler, consisting of a two-stage Mach-Zehnder interferometer with electro-optic phase shifters in each stage. We demonstrate that such design can be configured as a tunable coupler with arbitrary coupling ratio and with a uniform response over 50-nm spectral range around 1550 nm. The design is also tolerant to fabrication variations that affect the coupling ratios of the directional couplers.

Highlights

Photonic integrated circuits (PIC) integrate multiple optical functions on a chip, similar as in an electronic integrated circuit
We find that k(λ1) = 0.75 and k(λ2) = 0.15 are corresponding coupling ratios of the directional couplers (DCs) at wavelengths λ1 = 1525 nm and λ2 = 1575 nm
We understand that we can configure the 2 × 2 × 2 Mach-Zehnder interferometer (MZI) as a wavelengthindependent tunable coupler, which is very useful for many applications such as wavelengthdivision multiplexing and signal switching in data communication

Summary

Introduction

Photonic integrated circuits (PIC) integrate multiple optical functions on a chip, similar as in an electronic integrated circuit. Due to fabrication variation, the fabricated device (e.g. 50:50 splitters/combiners) will deviate from the designed value, and this will limit the coupling range that can be addressed with the MZI circuit. A fabrication-tolerant 2 × 2 coupler circuit, that can be tuned from 0 to 100% coupling, and this over a wide wavelength range, is a highly needed building block to construct complex circuits. Even if the directional couplers have a perfect 50:50 splitting ratio, this is usually only the case for a narrow wavelength range, as the directional couplers are wavelength dependent Alternative couplers, such as MMIs [8] and phase-compensated directional couplers [5] offer a more broadband 50:50 coupler, but these can suffer from deviations due to fabrication variability. Compared to the three-stage design, the tuning algorithm becomes simpler while it is still tolerant to fabrication imperfections and even local variability

Theoretical analysis

Tuning for broadband operation

Experimental results

Optimization algorithm

Gradient descent algorithm

Conclusion