On-chip 7 GHz acousto-optic modulators for visible wavelengths

TL;DR

The paper addresses the need for compact, high-frequency visible-light modulators by introducing a chip-scale acousto-optic modulator operating at $7\, \mathrm{GHz}$ on a lithium niobate on sapphire platform. The device achieves phase modulation in a 200 µm interaction length by guiding SAWs perpendicular to a wedge LN waveguide, enabling efficient on-chip modulation at visible wavelengths and revealing a notable sideband asymmetry. A multimode interaction framework involving TE$_0$ and TM$_0$ modes explains the observed asymmetry via a nonzero modulation phase delay $\phi_0$, and suggests routes to achieve single-sideband modulation through geometric and phase-control parameters. This work advances integrated photonics for quantum and sensing applications by enabling compact, high-frequency, polarization-aware AOMs compatible with atomic platforms and visible-light photonics.

Abstract

A chip-integrated acousto-optic phase modulator tailored for visible optical wavelengths has been developed. Utilizing the lithium niobate on sapphire platform, the modulator employs a 7 GHz surface acoustic wave, excited by an interdigital transducer and aligned perpendicular to the waveguide. This design achieves efficient phase modulation of visible light within a compact device length of merely 200 microns, while holds the advantages of easy fabrication and high stability due to simple unsuspended structure. Remarkably, in this high-frequency acoustic regime, the acoustic wavelength becomes comparable to the optical wavelength, resulting in a notable single-sideband modulation behavior. This observation underscores the phase delay effects in the acousto-optics interactions, and opens up new aspects for realizing functional visible photonic devices and its integration with atom- and ion-based quantum platforms.

Figures (6)

• Figure 1: (a) Schematic diagram of the integrated AOM on the thin-film lithium niobate platform. The substrate is sapphire, and the metal electrodes are made of aluminium. X, Y, Z represents the crystal axes of lithium niobate. $\mathrm{X^{'}}$, $\mathrm{Y^{'}}$, $\mathrm{Z^{'}}$ represent the crystal axes of thin-film sapphire. (b) 2D cross-section of the integrated AOM, illustrating the electrode fingers of the IDT for exciting SAW, with the period of the electrode fingers is $L_{\mathrm{pitch}}$. (c) The distributions of strain field ($S_{\mathrm{XX}}$) induced by the SAW. (d) The electric field profiles of fundamental transverse-electric mode at the cross-section of the waveguide. (e) Photograph of the fabricated AOMs on the chip, featuring array of IDTs. (f) Optical micrograph of the AOM device, highlighting the IDT with electrode fingers parallel to the optical waveguide.
• Figure 2: (a) The optical micrograph and the SEM image of IDT pair. The length of each IDT is 200µm, and the number of electrode pair is 20. The distance between two IDTs is 80µm. From the SEM image of IDT, the width of each electrode is about 196nm, and the distance between two adjacent electrode is about 345nm. The scale bar of the SEM image is 2µm. (b) The S21 transmission spectra of several IDT pairs with different electrode pair number. (c) and (d) The experimental results (Theoretical results) of conversion efficiency $\eta_{\mathrm{IDT}}$ (RF to SAW) and the bandwidth $\Delta_{\mathrm{IDT}}$ versus electrode pair number $N_{\mathrm{IDT}}$.
• Figure 3: (a) Schematic diagram of the experiment setup. The heterodyne beat measurement is used to measure the modulation efficiency of each sideband of our device. Laser : 780nm. PC : Polarization Controller. Commercial AOM : bulk acousto-optic modulation, the center frequency is 110MHz. It's used to tune the frequency of the local oscillator. DUT : device under test. PD : a high-speed photodetector. ESA : the electric spectrum analyzer. (b) These graphs are relative optical amplitude of carrier, the positive first-order sideband and the negative firest-order sideband, respectively. The resolution bandwidth (RBW) is 39Hz, corresponding to the noise level which is about -128dBm. (c) The experimental results of the S21 transmission spectra of the on-chip IDT pair and modulation efficiency of each sideband versus the microwave frequency. The microwave's drive power is about 30dBm.
• Figure 4: Polarization-dependent sideband asymmetry in the AOM. (a) and (b) Measured power of the carrier (blue), $+1$ sideband (red), and $-1$ sideband (yellow) for different polarization states of the local oscillator light in the heterodyne detection setup. The LO polarization is varied by adjusting polarization controller (PC3), and the results in (a) and (b) are obtained with different input polarizations by adjusting the PC1 to minimize and maximize the carrier transmission, respectively. (c) Control experiment using a commercial fiber-coupled electro-optic modulator (EOM), with the input polarization is adjusted for maximal transmission.
• Figure 5: Multimode acousto-optic interaction and phase-delay effects. (a) Schematic of the proposed multimode acousto-optic modulation model, incorporating the fundamental $\mathrm{TE}_{0}$ and $\mathrm{TM}_{0}$ modes. The mode mixers account for the excitation and interference of the two modes, while the acousto-optic interaction induces phase modulation with a relative delay ($\phi_0$) between the modes. (b) Numerically calculated acousto-optic coupling strength for $\mathrm{TE}_{0}$ (blue) and $\mathrm{TM}_{0}$ (red) as a function of waveguide width. The black curve shows the phase difference between the coupling strengths. (c) Profiles of the strain field ($S_{\mathrm{XX}}$) for phase delays of 0 and $-0.72\pi$, respectively. (d)-(e) Real and imaginary parts of the refractive index modulation $\zeta$ for the $\mathrm{TE}_{0}$ and $\mathrm{TM}_{0}$ modes, respectively, at zero phase dealy. (f) Real and imaginary parts of $\zeta$ for $\mathrm{TM}_{0}$ mode at a phase delay for $\phi=-0.72\pi$.