An integrated multi-THz tunable linear isolator based on electro-optic non-reciprocal strong coupling

Abstract

Optical isolators are essential for laser protection and robust signal routing, but the incorporation of the necessary magneto-optic (MO) materials in foundries has remained a challenge. As an alternative, several integrated non-magnetic isolators based on acousto-optic (AO) and electro-optic (EO) spatio-temporal modulation have been proposed. Unlike MO isolators, these solutions are wavelength agnostic, though few published demonstrations reach performance that is comparable to MO devices. The most significant remaining concerns are on mitigating undesirable sidebands, achieving wide bandwidth or wide tunability, and having a design that is practical to deploy. Most of these challenges can be addressed through non-reciprocal strong coupling between waveguide or resonator modes, with the intent to produce extremely asymmetric optical dispersion, but this has never been accomplished with electro-optics. Here we demonstrate a compact EO optical isolator, using thin film lithium niobate, that is the first EO device to reach the non-reciprocal strong coupling regime. In this new regime, the isolator produces a very high isolation figure of merit ($>32$ dB contrast per dB of insertion loss, 47.7 dB isolation contrast with 1.45 dB insertion loss) and, due to its architecture, achieves linear operation with negligible sideband generation. We additionally demonstrate THz-scale (8 nm) tunability of the isolation band that is not fundamentally limited, and can be extended to multi-THz operation.

Figures (8)

• Figure 1: Isolator design and operational principle.(a) The isolator is composed of a bus waveguide coupled to a chiral photonic resonator whose optical density of states is made strongly asymmetric using an electro-optic (EO) modulation. Light propagating in the forward direction along the waveguide experiences no interaction with the resonator, while simultaneously light propagating in the backward direction is resonantly absorbed. (b) This energy-momentum diagram shows the optical modes of the resonator configured to be separated in both frequency and momentum space, forming a two-level photonic atom. The EO stimulus generates coupling rate $G_{eo}$ for one direction of circulation, resulting in a strongly chiral DoS due to photonic Autler-Townes splitting (p-ATS). (c) This illustration conveys that the RF stimulus must be designed with odd symmetry to help break the orthogonality between the optical modes of the resonator.
• Figure 1: Heterodyne detection system used for measuring the carrier and sidebands. ECDL = External cavity diode laser. DUT = Device under test. FPC = Fiber polarization controller. AOFS = Acousto-Optic Frequency Shifter. PD = Photodetector.
• Figure 2: Design and implementation of the electro-optic isolator.(a) The cross-sectional schematic of the double-racetrack resonator shows the lithium niobate (LN) waveguide structure (dark blue), electrode configuration (yellow/orange), and oxide cladding (grey). (b) Finite element simulation of the TEeven and TEodd optical modes supported by the resonator. (c) Finite element simulation of the RF mode showing the transverse mirror symmetric electric-field profile. (d) A plan-view schematic of the 3-phase EO stimulus section is presented. The split-electrode structure is driven through three RF bus lines with $120^{\circ}$ relative phase shift with matched amplitudes at driving frequency $\Omega$. (e) A top-view optical microscope image of the fabricated EO isolator. Upper section of the racetrack resonator shows the 3-phase EO modulator with the inset showing a zoomed-in view of a single electrode. Lower section of the resonator enables EO fine-tuning of the isolator.
• Figure 2: Measurement of the RF reflection coefficient (S11) at the electrodes. The slight dip observed around 1.5 GHz arises from resonance effects in the cables, attributed to back-reflections from the electrode.
• Figure 3: Experimental demonstration and characterization of isolation.(a) Transmission measured through the waveguide shows the two optical mode families hosted by the resonator. We chose the optical mode pair marked by $\color{red}\bm{\star}$ located near 193.98 THz (1545.5 nm) to conduct our experiments. (b) With RF stimulus set to $\Omega = 4.8$ GHz, which matches the modal frequency gap, we measured evolution of the through-waveguide transmission for the $\color{red}\bm{\star}$ mode pair. As the applied RF voltage (VRF) is increased, the p-ATS phenomenon appears for forward light propagation direction while the backward direction remains strongly absorbing. (c) We present a detailed view of the superimposed forward and backward measurements for the 15.88 V case, corresponding to a measured $G_{eo}$ of approximately 0.89 GHz.