Multiband Electro-Optic Modulator Employing a Lithium Niobate Racetrack Resonator Integrated with Two Pulley Couplers

Integrated optical modulators (IOMs) have attracted attention in various applications, including optical interconnects, sensors, spectroscopic devices, and quantum photonic circuits. The visible to near-infrared wavelength IOMs exhibit potential for the manipulation of the ion or neutral atom qubits, while the telecommunication wavelength IOMs play a crucial role for the development of quantum photonic circuits and long distance communication. It has thus been relevant to address and bridge the elements operating in multiple spectral bands with a single IOM. In this study, we present a multiband electro-optic (EO) modulator employing a lithium niobate (LN) micro-resonator integrated with two pulley couplers. Our device offers high modulation extinction ratios over multiple spectral bands at 775, 980, and 1550 nm wavelengths. The EO modulator consists of a high-quality-factor racetrack micro-resonator and two pulley couplers. While the first coupler divides the incident light into two paths along the resonator and the bus waveguide, the second coupler recombines the traveling light. In the two-coupler integration method, the external coupling strength is tunable sinusoidal way depending on the relative phase difference of the two paths, respectively. When the four times of the coupling strength at the one-coupler (κ_one) exceeds the intrinsic loss of the resonator (κ_int), the high-extinction EO modulation traversing from the under, through the critical to the overcoupling states is allowed. We should note that the multiband operation originates from the synergy of several essential ingredients, including the wide transparent window of LN, the high quality factor of the racetrack resonator, the aforementioned advantage of the two-coupler integration method, and the broadband performance of the pulley coupler. In particular, we designed the pulley coupler by adjusting the structural parameters such as the gap size, waveguide width, and interaction angle to support sufficiently high coupling strength even at short wavelengths. Both the theoretical and experimental results confirm that the condition of 4κ_one > κ_int, which is required for high-extinction EO modulation, is satisfied in all the targeted wavelength bands. The applied voltage-dependent transmission spectra were precisely characterized in the multiple bands using an experimental set-up consisting of three different external-cavity diode lasers and Mach-Zehnder interferometers. The experimental results for representative resonant modes in each spectral band demonstrate that the EO transition from the under-, the critical- to the over-coupling condition was realized. The extinction ratio at the critical-coupling condition reached 12 dB at the wavelength of 775.2 nm, 10 dB at 969.7 nm, and 21 dB at 1539.5 nm. The effective half-wave voltage was evaluated from the EO evolution of the transmittance at the critical-coupling frequency: approximately 1.0, 3.7, and 2.7 V at the wavelengths of 1539.5, 969.7, and 775.2 nm. The length of the EO modulation part of our device was 2.5 mm, and then the half-wave voltage-length product was evaluated as 0.25, 0.93, and 0.68 V×cm. This value in the telecommunication band is an order of magnitude lower than those obtained in typical MZI-based IOMs, owing to the high quality factor of the LN racetrack micro-resonator. In summary, our multiband IOM based on the thin-film LN micro-resonator and the two-pulley coupled platform represents the widest operating spectral range in a single device to our knowledge, and is ready to extend to midinfrared wavelengths, where LN is still highly transparent. The multiband EO modulator holds promise for low-cost, high-density integration in optical communications and quantum information processing.