Acoustic Arrayed Waveguide Grating (AAWG)

• Acoustic Arrayed Waveguide Grating (AAWG) is an integrated device that routes acoustic signals in the frequency domain using dispersion-engineered, incremental waveguide arrays.
• It enables MHz-level channel separation with robust isolation, achieving 3.8 MHz resolution per channel and minimal insertion loss in compact GaN-based platforms.
• Implementations utilize precision waveguide tapering and phase control to support scalable, programmable phononic integrated circuits for advanced RF and quantum applications.

Acoustic Arrayed Waveguide Grating (AAWG) is an integrated wave-based device that performs frequency-domain spatial routing of acoustic (phonon) signals by exploiting dispersion-engineered path length gradients in an array of parallel acoustic waveguides. Analogous to the arrayed waveguide grating (AWG) in photonic systems, the AAWG serves as a frequency demultiplexer, enabling spatial separation of different frequencies with high channel density, MHz-level selectivity, and robust isolation between outputs. Recent research has demonstrated large-scale programmable phononic integrated circuits incorporating AAWGs as central elements for multiplexed acoustic signal processing (Xu et al., 30 Oct 2025).

1. Architecture and Fundamental Operating Principles

AAWG devices are constructed on high acoustic index-contrast platforms, notably gallium nitride (GaN) on sapphire, which enable tight confinement of gigahertz-frequency phonons and support device miniaturization. The canonical AAWG consists of two free propagation regions (FPRs) flanking an array of $N$ parallel waveguides, each incrementally longer by $\Delta L$ than its neighbor. An acoustic input enters FPR1, is distributed across the array, traverses the differentially phased waveguides, and the outputs recombine in FPR2, where they focus to spatially distinct output ports. The frequency-dependent accumulated phase in each arm dictates the constructive interference location in FPR2, yielding spatial separation of individual frequency components.

The core routing condition is given by the acoustic grating equation:

$$\beta_{s}(f) d \cdot \theta_{out,n} + \Delta\phi(f) = 2\pi m$$

where $\beta_{s}(f)$ is the slab propagation constant at frequency $f$, $d$ is the output waveguide pitch, $\theta_{out,n}$ is the angular position of the $n$-th output port, and $m$ is the diffraction order.

2. Acoustic Wave Propagation, Polarization, and Dispersion Engineering

PnIC platforms enable propagation of quasi-Rayleigh (out-of-plane) and quasi-Love (in-plane) modes at GHz frequencies. Key functional elements—waveguides, splitters, directional couplers, and polarization converters—control modal distribution and dispersion. In the AAWG, dispersion engineering via waveguide geometry and material parameters determines both the phase gradient $\Delta\phi(f) = \beta(f) \Delta L$ and the free spectral range (FSR):

$\mathrm{FSR} = \frac{v_g}{\Delta L}$

where $v_g$ is the group velocity for the guided acoustic mode. Channel resolution is tuned by the number of outputs:

$\Delta f = \frac{v_g}{N_{out} \Delta L}$

Polarization conversion is implemented by adiabatic tapers inducing mode hybridization at avoided crossings, providing selective transfer between Rayleigh and Love modes for advanced signal encoding and routing.

3. Implementation and Performance Metrics

AAWG devices have been experimentally realized with $N_{out}=21$ output ports, each corresponding to a 3.8 MHz wide frequency channel, centered near 1.5 GHz. The measured free spectral range is 81 MHz, matching theoretical predictions. Channel isolation exceeds 10 dB for nearest neighbors and 20 dB for next-nearest, substantiating robust frequency selectivity.

Insertion loss of a complete AAWG device, inclusive of combiners and ancillary routing, is $\sim$4.5 dB. Integration density is high; the physical area of the demultiplexer is on the scale of a millimeter squared, comparable to photonic AWG implementations. The critical performance metrics are summarized below.

Device	Channel Count	Resolution	Insertion Loss	Channel Isolation
AAWG	21	3.8 MHz/channel	4.5 dB	10–20 dB

Uniformity and scalability are further evidenced by the implementation of a 1×128 power splitter, which distributes phonon signal nearly evenly to 128 outputs using cascaded Y-splitters, achieving $\sim$0.5 dB loss per splitter and an overall integration density of 3,300/cm².

4. Comparison to Photonic AWG Devices and Theoretical Context

The operational principle of AAWGs is directly analogous to photonic AWG devices: path-length engineered arrays induce frequency-dependent phase shifts, resulting in spatial frequency demultiplexing (Stoll et al., 2021). In both modalities, the use of FPRs for mode expansion and collection, phased arrays for dispersion-induced routing, and geometry optimization for crosstalk and aberration mitigation are central design aspects.

Whereas photonic AWGs achieve channel spacings of tens to hundreds of GHz for telecom applications, AAWGs operate in the MHz regime, adapted for acoustic wavelengths and group velocities. Aberration, phase error, and polarization sensitivity are equally critical in both domains; in AAWGs, they manifest as loss, nonuniformity, and incomplete channel separation, necessitating advanced fabrication and design strategies.

5. Applications and System-level Integration

AAWGs, as demultiplexers, are foundational for multi-channel acoustic signal processing, real-time spectrum analysis, and programmable spectral filtering in PnICs. Integration with active thermoacoustic phase modulators and additional passive components (interferometers, ring resonators) enables complex functions such as reconfigurable frequency synthesizers, high-fidelity signal routing, and spectral logic operation.

AAWGs permit MHz-resolution channelization across tens of outputs at high integration densities, supporting hybrid photonic-electronic-phononic chips for advanced RF, classical, and quantum information processing.

6. Physical Mechanisms and Engineering Considerations

The performance of AAWGs is dictated by the precision of waveguide geometry ($\Delta L$), acoustic mode selection, material quality (loss, dispersion), and the quality factor of the output channels. Loss scaling is determined primarily by propagation loss in the GaN waveguides (measured at 2.4 dB/mm). Limiting factors for further scaling include cumulative insertion loss and uniformity degradation due to phase errors and fabrication tolerances.

Phase control, polarization conversion, and dispersion engineering are realized through detailed waveguide shaping and thermal modulation; programmable operation is enabled by integration of modulator elements that tune local phase shifts via the thermoacoustic effect.

7. Outlook and Implications

The demonstration of AAWGs with MHz channel spacing, high isolation, and massive port count establishes a fundamental advance in the field of integrated phononics. The analogy to photonic AWGs enables cross-domain design methodologies, and the capacity for large-scale AAWG integration on GaN platforms points to future avenues in RF signal processing, quantum acoustics, and multi-domain information processing chips.

A plausible implication is that continued reduction of waveguide loss and improved phase/polarization control could enable AAWG devices with hundreds of frequency channels, full programmability, and quantum-grade isolation, serving as central technology for next-generation phononic, hybrid, and quantum integrated circuits (Xu et al., 30 Oct 2025).