Chip-Integrated Brillouin Saser Gyroscope

• Chip-integrated Brillouin saser gyroscope is a monolithic sensor that exploits co-confined optical and acoustic modes in an LN on sapphire platform for precise inertial sensing.
• It utilizes backward stimulated Brillouin scattering to generate both laser and saser outputs, achieving superior noise suppression and reduced angle random walk at moderate pump powers.
• The integration of optical and electrical readouts in a compact microring resonator supports advanced applications in quantum transduction, RF signal processing, and precision metrology.

The chip-integrated Brillouin saser gyroscope employs opto-acoustic interaction within a monolithically fabricated platform to achieve highly sensitive rotation detection. Unlike conventional Brillouin laser gyroscopes, which only leverage optical readout, the chip-integrated saser variant captures the simultaneously generated sound amplification by stimulated emission of radiation (“saser”) output through direct acoustic detection. This scheme provides superior noise suppression and reduced angle random walk (ARW), enabled by innovative integration strategies that confine both optical and acoustic modes in a lithium niobate on sapphire (LNOS) stack. Accessible acoustic output facilitates advanced functionality in inertial sensing, quantum transduction, and RF signal processing, with competitive metrics attainable at substantially reduced power thresholds and device complexity (Duan et al., 20 Nov 2025).

1. On-Chip Architecture and Mode Confinement

The “Zhengfu” chip-integrated Brillouin saser gyroscope is implemented in a thin-film lithium niobate (LN) on sapphire (LNOS) substrate. High acoustic velocity contrast between LN and sapphire allows rigorous confinement of both the optical whispering-gallery mode (WGM) at $\lambda \approx 1550\,\mathrm{nm}$ ($$\omega_{\mathrm{opt}}/2\pi \approx 193\,\mathrm{THz}$$) and the backward-Brillouin acoustic mode at $\Omega_{\mathrm{aco}}/2\pi \approx 9\,\mathrm{GHz}$ within a microring resonator of radius $R\sim 50$–$200\,\mu\mathrm{m}$. Absence of suspended waveguides is enabled by the single-crystal LNOS stack, which suppresses acoustic leakage and achieves acoustic quality factors $Q_{\mathrm{aco}}$ up to several $10^3$ without substrate etching or undercuts.

Integrated phonon waveguides and interdigital transducers (IDTs) directly access the saser signal as an electrical output, while a bus optical waveguide simultaneously serves the Brillouin laser output. The co-location of electrical and optical readout in a compact, monolithic chip sets the foundation for direct phononic-electronic optical interfaces at microwave frequencies.

Property	Optical Mode	Acoustic Mode
Frequency	$\omega_{\mathrm{opt}}/2\pi$ $\sim$ 193 THz	$\Omega_{\mathrm{aco}}/2\pi$ $\sim$ 9 GHz
Quality Factor ($Q$)	$Q_{\mathrm{opt}}\sim 10^5$–$10^6$	$Q_{\mathrm{aco}}\sim 5\times 10^3$
Readout Port	Bus waveguide (optical)	Phonon waveguide + IDT (electrical)

2. Brillouin Gain, Saser Threshold, and Oscillation Dynamics

Operation rests on backward stimulated Brillouin scattering (SBS), described by the Hamiltonian

$$H_{\mathrm{int}} = \hbar\,g_0\,\left(a_p^\dagger a_{\mathrm{aco}} a_s + a_p a_{\mathrm{aco}}^\dagger a_s^\dagger\right)$$

where $a_p$, $a_s$, $a_{\mathrm{aco}}$ represent pump, Stokes, and acoustic mode operators; $g_0$ is the single-photon Brillouin coupling rate ($\sim 2\pi \times 10-10^2\ \mathrm{Hz}$ in LNOS). Cooperativity is given by

$$C = \frac{N_p g_0^2}{\kappa_{\mathrm{opt}} \kappa_{\mathrm{aco}}}$$

with $N_p$ the intracavity photon number, $$\kappa_{\mathrm{opt}} = \omega_{\mathrm{opt}}/Q_{\mathrm{opt}}$$, and $$\kappa_{\mathrm{aco}} = \Omega_{\mathrm{aco}}/Q_{\mathrm{aco}}$$. Threshold for simultaneous laser and saser oscillation is reached at $C \rightarrow 1$, i.e.,

$$N_{p,\mathrm{th}} = \frac{\kappa_{\mathrm{opt}} \kappa_{\mathrm{aco}}}{g_0^2}$$

corresponding in practice to pump powers of a few mW at $Q_{\mathrm{opt}}\approx 10^5$, $Q_{\mathrm{aco}}\approx 5\times 10^3$.

3. Quality Factors, Linewidths, and Noise Performance

Quality factor ($Q$) critically affects noise and stability:

• $$Q_{\mathrm{opt}} = \omega_{\mathrm{opt}}/\kappa_{\mathrm{opt}} \sim 10^5$$–$10^6$ ($\kappa_{\mathrm{opt}}/2\pi \sim 2$–$20\,\mathrm{MHz}$),
• $$Q_{\mathrm{aco}} = \Omega_{\mathrm{aco}}/\kappa_{\mathrm{aco}} \sim 5\times 10^3$$ ($\kappa_{\mathrm{aco}}/2\pi \sim 1.8\,\mathrm{MHz}$).

Intrinsic thermal-limited linewidth (Schawlow–Townes term) is

$$\Delta\nu_0 = \frac{\kappa_{\mathrm{opt}}\kappa_{\mathrm{aco}}}{(\kappa_{\mathrm{opt}}+\kappa_{\mathrm{aco}})^2}\frac{1}{4\pi N_{\mathrm{aco}}}$$

where $N_{\mathrm{aco}}$ (intracavity phonon number) is $10^7$–$10^9$. Pump-noise transfer to laser or saser channels is

$$\Delta\nu_{\mathrm{laser}}^{\mathrm{pump}} = \left(\frac{\kappa_{\mathrm{opt}}}{\kappa_{\mathrm{opt}}+\kappa_{\mathrm{aco}}}\right)^2 \Delta\nu_p,\qquad \Delta\nu_{\mathrm{saser}}^{\mathrm{pump}} = \left(\frac{\kappa_{\mathrm{aco}}}{\kappa_{\mathrm{opt}}+\kappa_{\mathrm{aco}}}\right)^2 \Delta\nu_p$$

with $\Delta\nu_p$ the pump laser linewidth ($\sim 1$ kHz). In saser regime ($Q_{\mathrm{aco}} \gg Q_{\mathrm{opt}}$), pump noise transferred to the saser is strongly suppressed, while the thermal-limited linewidth is minimized through large $N_{\mathrm{aco}}$ (Duan et al., 20 Nov 2025).

4. Rotation Detection and Angle Random Walk Analysis

Rotation $\Omega$ is detected by beating CW and CCW saser outputs; the beat frequency is

$$\nu_{\mathrm{beat}} = \frac{S_{\mathrm{eff}}|\Omega|}{2\pi}$$

where the effective Sagnac scale factor is

$$S_{\mathrm{eff}} = \frac{\kappa_{\mathrm{aco}} S_{\mathrm{opt}} + \kappa_{\mathrm{opt}} S_{\mathrm{aco}}}{\kappa_{\mathrm{opt}}+\kappa_{\mathrm{aco}}},$$

and $$S_{\mathrm{opt}} = (\omega_{\mathrm{opt}} R) / v_{\mathrm{opt}}$$, $$S_{\mathrm{aco}} = (\Omega_{\mathrm{aco}} R) / v_{\mathrm{aco}} \approx 2S_{\mathrm{opt}}$$.

Angle random walk (ARW) is derived as

$$\mathrm{ARW} = \sqrt{\frac{\Delta\nu_{\min}/\pi}{S_{\mathrm{eff}}^2}}$$

where $\Delta\nu_{\min}$ is the minimal linewidth (laser or saser channel), producing ARW $\sim 0.1\,\mathrm{deg}/\sqrt{\mathrm{h}}$ in practical chip designs at moderate pump powers (e.g., $P=200\,\mathrm{mW}$, $Q_{\mathrm{aco}} \sim 5\times 10^3$).

5. Comparative Performance: Saser vs. Conventional Brillouin Laser Gyroscopes

Conventional Brillouin laser gyroscopes operate in $Q_{\mathrm{aco}}\ll Q_{\mathrm{opt}}$ regime, requiring $50$–$200\,\mathrm{mW}$ pump powers and high optical quality factors ($Q_{\mathrm{opt}}>10^8$–$10^9$) to reach sub-$1\,\mathrm{Hz}$ linewidth. Saser gyroscopes, at $Q_{\mathrm{aco}}\sim 5\times 10^3$, $Q_{\mathrm{opt}}\sim 10^5$–$10^6$, achieve:

• $\Delta\nu_{\mathrm{saser}}\approx 0.5\,\mathrm{Hz}$ ($$\mathrm{ARW}\approx0.5\,\mathrm{deg}/\sqrt{\mathrm{h}}$$) at $5\,\mathrm{mW}$ pump,
• $$\Delta\nu_{\mathrm{saser}}\approx 0.02\,\mathrm{Hz}$$ ($$\mathrm{ARW}\approx0.085\,\mathrm{deg}/\sqrt{\mathrm{h}}$$) at $200\,\mathrm{mW}$ pump.

Achieving comparable ARW in a laser-only device would require $Q_{\mathrm{opt}}>10^{10}$ or pump powers exceeding $500\,\mathrm{mW}$ (Duan et al., 20 Nov 2025).

6. Mechanisms for Noise Suppression: Acoustic Detection Advantages

Acoustic detection provides dramatic suppression of pump frequency noise, as shown by

$$\Delta\nu_{\mathrm{saser}} = (n_{\mathrm{th}} + 1)\Delta\nu_0 + \left(\frac{\kappa_{\mathrm{aco}}}{\kappa_{\mathrm{opt}}+\kappa_{\mathrm{aco}}}\right)^2 \Delta\nu_p$$

By engineering $Q_{\mathrm{aco}} \gg Q_{\mathrm{opt}}$, $$(\kappa_{\mathrm{aco}}/(\kappa_{\mathrm{opt}}+\kappa_{\mathrm{aco}}))^2 \ll 1$$, minimizing the pump-noise term. Simultaneous increase in $Q_{\mathrm{aco}}$ yields a larger phonon occupation, lowering the intrinsic linewidth $\Delta\nu_0$ and further enhancing signal stability.

7. Design Guidelines and Applications

Transitioning into the saser regime necessitates $Q_{\mathrm{aco}} \geq 10^3$, deliverable through low-loss LNOS waveguides, and $Q_{\mathrm{opt}} \sim 10^5$–$10^6$. Increasing microring radius $R$ augments Sagnac scale ($S_{\mathrm{aco}}\simeq 2S_{\mathrm{opt}}$), but incurs trade-offs in free spectral range and pump threshold. Optimal IDT configuration is required for maximal $9\,\mathrm{GHz}$ phonon extraction.

Key application domains include:

• Quantum transduction: interfacing microwave (via piezoelectric IDT) and optical photons.
• RF signal processing: on-chip Brillouin amplifiers, filters, oscillators.
• Precision measurement: chip-scale frequency references and low-noise microwave sources.

A plausible implication is that saser-based approaches will underpin future active phononic integrated circuits with Brillouin gain, setting the stage for more compact, stable, and power-efficient inertial sensors and hybrid photonic-phononic platforms.

8. Context, Significance, and Outlook

The chip-integrated Brillouin saser gyroscope represents a convergence of phononic and photonic integration, utilizing direct acoustic readout to overcome limitations in frequency and thermal noise inherent to conventional optical-only gyroscopes. The capacity to engineer and access both high-$Q$ optical and acoustic modes without suspended architectures enables high-performance inertial sensing with ARW values down to $0.1\,\mathrm{deg}/\sqrt{\mathrm{h}}$ at tens of milliwatts pump power, which is orders of magnitude more efficient than legacy approaches. This platform provides transformative opportunities across quantum transduction, RF photonics, and precision metrology (Duan et al., 20 Nov 2025).

Further research will likely refine phonon extraction efficiency, increase scalability, and explore new modalities in hybrid quantum systems, expanding the role of integrated Brillouin gain mechanisms in next-generation sensor and transducer arrays.