[Paper Review] Inverse-designed photonic circuits for fully passive, bias-free Kerr-based nonreciprocal transmission and routing

Ki Youl Yang, Jinhie Skarda+7 more|arXiv (Cornell University)|May 13, 2019

Advanced Fiber Laser Technologies46 references50 citations

TL;DR

The paper demonstrates fully passive, bias-free Kerr-nonlinear nonreciprocal transmission and routing on silicon photonics, using inverse-designed Fano reflectors and cascaded Fano-Lorentzian resonators to achieve high forward transmission with broad NRIR, plus on-chip LIDAR demonstrations.

ABSTRACT

Nonreciprocal devices such as isolators and circulators are key enabling technologies for communication systems, both at microwave and optical frequencies. While nonreciprocal devices based on magnetic effects are available for free-space and fibre-optic communication systems, their on-chip integration has been challenging, primarily due to the concomitant high insertion loss, weak magneto-optical effects, and material incompatibility. We show that Kerr nonlinear resonators can be used to achieve all-passive, low-loss, bias-free, broadband nonreciprocal transmission and routing for applications in photonic systems such as chip-scale LIDAR. A multi-port nonlinear Fano resonator is used as an on-chip, all-optical router for frequency comb based distance measurement. Since time-reversal symmetry imposes stringent limitations on the operating power range and transmission of a single nonlinear resonator, we implement a cascaded Fano-Lorentzian resonator system that overcomes these limitations and significantly improves the insertion loss, bandwidth and non-reciprocal power range of current state-of-the-art devices. This work provides a platform-independent design for nonreciprocal transmission and routing that are ideally suited for photonic integration.

Motivation & Objective

• Motivate and enable on-chip nonreciprocal photonic devices without magnetic bias or external modulation.
• Design and demonstrate Kerr-based, all-passive nonreciprocal transmission and routing in silicon photonics.
• Relax the traditional single-resonator bandwidth-transmission trade-off via cascaded nonlinear resonators.

Proposed method

• Use photonics inverse design (SPINS) to create inverse-designed reflectors that generate Fano resonances in a silicon racetrack resonator.
• Implement asymmetric coupling to break forward/backward symmetry and achieve nonreciprocity in transmission spectra.
• Characterize single Fano resonator devices to verify the fundamental TNRIR bound.
• Cascade a Fano resonator with a Lorentzian resonator with lithographically controlled phase delay to broaden operation while maintaining high forward transmission.
• Demonstrate on-chip routing and an optical distance measurement (LIDAR) using the nonreciprocal device in a pulsed, frequency-comb-based setup.

Experimental results

Research questions

• RQ1 Can Kerr nonlinear silicon resonators achieve bias-free, all-passive nonreciprocity suitable for integrated photonics?
• RQ2 What is the practical performance (insertion loss, NRIR, bandwidth) of single-resonator Kerr nonreciprocal devices, and can it be improved via cascaded resonators?
• RQ3 Does inverse-design enable robust, broadband, low-loss nonreciprocity and routability for chip-scale applications such as LIDAR?
• RQ4 Can the device support high-speed, pulsed operation and be integrated into a functional optical ranging system?

Key findings

• A Fano nonreciprocal device on silicon achieves up to 24.7 dB forward-backward transmission contrast (min 13.4 dB), with 1.3 dB insertion loss and 3.9 dB NRIR within a 4.55–8.15 dBm loaded power range.
• A single-resonator device is experimentally shown to obey the fundamental bound T ≤ 4·NRIR/(NRIR+1)^2, limiting forward transmission vs NRIR.
• Cascading a Fano and a Lorentzian resonator with a controlled phase delay yields near-unity forward transmission (>99%) over NRIR > 6 dB, breaking the single-resonator bound.
• Broadband isolation and high forward transmission are demonstrated across cascaded resonators, with 0.04–0.22 dB insertion loss in some configurations.
• The device enables on-chip pulsed-radar/LIDAR-like ranging, achieving distance measurements up to 60 m using a frequency-comb based source, while protecting the pump from reflections.
• The approach provides a platform-independent design methodology for passive, bias-free nonreciprocal transmission and routing in silicon photonics, with potential integration for LIDAR and photonic signal processing.