Broadband acousto-optic modulators on Silicon Nitride

TL;DR

This work addresses the lack of optically broadband acousto-optic modulators on silicon nitride by introducing a traveling-wave spiral architecture that coherently multiplies the light-acoustic interaction length without altering the foundry stack. Utilizing 90 nm-thick SiN waveguides and AlN transducers, the authors demonstrate phase-matched modulation across spirals up to $L \approx 26~\mathrm{cm}$, achieving a peak $V_\pi$ of $8.98~\mathrm{V}$ at $704~\mathrm{MHz}$ with only $1.13~\mathrm{dB}$ insertion loss, and validate the coherence length concept with $L_\text{coh} \approx 13~\mathrm{cm}$. The approach yields broadband optical response with minimal wavelength dependence over a $90$ nm span and shows scalable modulation index growth with the number of modulated segments, aided by transducer impedance optimization that can deliver substantial gains. Importantly, the method preserves low optical losses and is compatible with commercial photonic foundries, enabling immediate deployment in on-chip optomechanical sensing and PDH-based readout while remaining adaptable to other low-loss PIC platforms.

Abstract

Stress-optic modulators are emerging as a necessary building block of photonic integrated circuits tasked with controlling and manipulating classical and quantum optical systems. While photonic platforms such as lithium niobate and silicon on insulator have well developed modulator ecosystems, silicon nitride so far does not. As silicon nitride has favorable optical properties, such as ultra-low-loss and a large optical transparency window, a rich ecosystem of potential photonic integrated circuits are therefore inhibited. Here we demonstrate a traveling wave optically broadband acousto-optic spiral modulator architecture at a wavelength of 1550 nm using 90 nm thick silicon nitride waveguides and demonstrate their use in an optomechanical sensing system. The spiral weaves the light repeatedly through the acoustic field up to 38 times, factoring in the time evolution of the acoustic field during the light's transit through spirals up to 26 cm in length. These modulators avoid heterogeneous integration, release processes, complicated fabrication procedures, and modifications of the commercial foundry fabricated photonic layer stack by exploiting ultra-low-loss waveguides to enable long phonon-photon interaction lengths required for efficient modulation. The design allows for thick top oxide cladding of 4 $μ$m such that the low loss optical properties of thin silicon nitride can be preserved, ultimately achieving a $V_π$ of 8.98 V at 704 MHz with 1.13 dB of insertion loss. Our modulators are the first optically broadband high frequency acousto-optic modulators on thin silicon nitride, and the novel architecture is accessible to any low loss photonic platform. We demonstrate an immediate use case for these devices in a high-Q optomechanical sensing system.

Figures (5)

• Figure 1: Overview of the modulator implementation and design | a. A illustration of the ultra-low-loss foundry-fabricated thin silicon nitride layer stack and subsequently deposited and fabricated piezoelectric transducer layers. The silica cladding above the waveguide is rendered transparent in the illustration so the waveguide is visible. b. A false colored cross-section scanning electron microscope image of the layer stack illustrated in (a). c. Simulation of the relevant strain tensor components, $S_{xx}$ and $S_{yy}$ for the fundamental transducer acoustic mode. d. An optical mode profile with arrows indicating the electric field is mostly $\hat{x}$-directed. e. An illustration of the design strategy. The $x$ location of the actively modulated waveguide sections are calculated such that the modulation is coherent in all modulated segments (straight waveguide sections) and design rules such as minimum bending radii are obeyed. f. The expected behavior of the spiral designs presented in this work. Modulation index is expected to trend linearly with respect to modulated segment count even beyond the coherence length. g. Dispersion diagram illustrating the effect of group delay compensation in the design of the spiral. By placing waveguides in the spiral according to the proposed method, phase matching can be achieved. In general, most spiral geometries will not satisfy phase matching, and performance will be sub-optimal, limiting modulation indices achievable.
• Figure 2: Validation of the modulator design |a. A chip without AlN deposited so the photonics are visible (left) and a completely fabricated chip with AlN and transducers (right). b. An optical microscope image of a transducer next to a spiral. There are 19 waveguides visible on the transducer side of the spiral, corresponding to $C=38$ modulated segments when counting both sides. The image color has been modified such that the waveguides are more easily distinguishable. c.$|S_{11}|^2$ of the transducer revealing electro-acoustic coupling. d. Modulation efficiency as a function of transducer RF drive frequency. The fringing pattern validates that both the near and far side of the spiral relative to the transducer both contribute to the modulation. e. Scaling of spiral geometric parameters as a function of modulated segment count. Note that the spiral area is defined as the area of the smallest rectangle it can fit in. f. Multiple spirals of varying pass count, $C$, are measured with identical transducer designs to ensure behavior follows the expected linear trend presented in Figure \ref{['fig:fig1']}c. g. Broadband modulation efficiency as a function of wavelength, showing $1.1dB$ variation across a $90nm$ range. h. Scaling of the modulation efficiency, for the first and second sidebands with respect to applied incident RF power, following the expected trend for acousto-optic modulation. The negative sidebands track identically to their positive counterparts.
• Figure 3: Higher-order acoustic modes |a.$S_{yy}$ strain tensor components for three other acoustic modes that can be excited by the transducer. Note that the waveguide location is marked, but in actual devices the waveguides do not reside under the transducers. b. A broadband RF drive frequency sweep revealing higher order acoustic modes coupling to the optical mode. c. Modulation efficiency for the $704MHz$ mode. It is the strongest performing acoustic mode due to the favorable overlap of the $S_{yy}$ strain tensor component with the waveguide shown in (a). However, it requires tight layer thickness control. d. Theoretical calculations illustrating relative degradation of modulation efficiency as acoustic mode frequency differs from the spiral's designed frequency. The shaded blue region shows the $3dB$ range, indicating all acoustic modes in (a) and (b) are still expected to nearly satisfy phase matching considerations. e. Theoretical calculation showing the range of possible modulation index trends expected across the $3dB$ shaded region in (d) as a function of modulated segments. The $704MHz$ mode in (f) exhibits behavior similar to the lower dashed black line. f. Scaling of the modulation index for the $704MHz$ contrasted against the fundamental mode. The agreement of the theory fits to the data validates the behavior claimed in (d). g. Modulation index scaling of the $502MHz$ and $639MHz$ modes. The modulation index scales nearly linearly, as expected from theory.
• Figure 4: Optimization of acousto-optic impedance match |a. An illustration of transducers with thickened bus bars. The layer stack is otherwise identical Fig. \ref{['fig:fig1']}a. b. An optical microscope image of the new transducer design with relevant geometric parameters denoted. c. The $|S_{11}|^2$ of these transducers. This subfigure can be directly compared to Figure \ref{['fig:fig2']}c to see a greater fraction of incident power is dissipated into the acoustic wave. d. Modulation index for all acoustic modes present, showing a substantial performance increase when compared to Figure \ref{['fig:fig2']}b.
• Figure 5: Application of modulators to optomechanical sensing system |a. A picture of an optomechanical accelerometer. Four optical ring resonators are accessible via packaging, with two being released and sensitive to acceleration. The chip was produced with an identical photonic layer stack to Figure \ref{['fig:fig1']}a (however, no piezoelectric was deposited). b. An optical microscope image of the SiO$_2$ serpentine spring structure that allows the ring resonator to be sensitive to acceleration. c. A diagram explaining device operation. As the proof mass displaces, the ring resonator is perturbed, shifting the optical resonance frequency. d. The experiment used to characterize the optomechanical accelerometer. e. Using $0.25W$ of RF power, a PDH error signal can be generated using the modulators. f. The transfer function and noise floor is measured. The fundamental resonance of the released mechanical structure is near $1.8kHz$.