[Paper Review] Enhanced sensitivity operation of an optical gyroscope near an exceptional point

Yu-Hung Lai, Yu‐Kun Lu+2 more|arXiv (Cornell University)|Jan 24, 2019

Advanced Fiber Laser Technologies68 citations

TL;DR

The authors experimentally demonstrate enhanced rotation sensitivity in a chip-based Brillouin gyroscope by operating near a second-order exceptional point, achieving over a 4× boost in Sagnac transduction.

ABSTRACT

Exceptional points (EPs) are special spectral degeneracies of non-Hermitian Hamiltonians governing the dynamics of open systems. At the EP two or more eigenvalues and the corresponding eigenstates coalesce. Recently, it has been proposed that EPs can enhance the sensitivity of optical gyroscopes. Here we report measurement of rotation sensitivity boost by over 4X resulting from operation of a chip-based stimulated Brillouin gyroscope near an exceptional point. A second-order EP is identified in the gyroscope and originates from the dissipative coupling between the clockwise and counterclockwise lasing modes. The modes experience opposing Sagnac shifts under application of a rotation, but near the exceptional point new modal admixtures dramatically increase the Sagnac shift. Modeling confirms the measured enhancement. Besides the ability to operate an optical gyroscope with enhanced sensitivity, this result provides a new platform for study of non-Hermitian physics and nonlinear optics with precise control.

Motivation & Objective

• Motivate study of non-Hermitian physics and exceptional points (EPs) in on-chip gyroscopes.
• Demonstrate enhanced rotation sensitivity by operating a dual counter-pumped Brillouin laser system near an EP.
• Identify and characterize a second-order EP arising from dissipative coupling between counter-propagating lasing modes.
• Quantify the Sagnac transduction factor and compare with theoretical predictions.
• Assess practical considerations such as Kerr shifts and noise on EP-assisted sensing.

Proposed method

• Use a high-Q silica wedge microresonator to implement a dual counter-pumped stimulated Brillouin laser (SBL) gyroscope.
• Model the CW/CCW mode interaction with a non-Hermitian Hamiltonian that includes dissipative coupling κ and Brillouin gain g_j.
• Derive the eigenfrequencies ω_s± and the SBL beating frequency Δω_s, including the square-root dependence near the EP (Δω_p ≈ Δω_c).
• Introduce pump detuning Δω_p as a control to pull the Stokes modes toward the gain center and to locate the EP.
• Measure dual-SBL beating spectra and Sagnac-induced shifts to extract the Sagnac transduction factor S and compare with theory (Eq. 6).
• Address Kerr-induced shifts and ensure they do not mask EP-enhanced transduction.

Experimental results

Research questions

• RQ1 Can exceptional points in a microresonator laser gyroscope enhance the transduction of rotation signals?
• RQ2 What is the nature of the eigenmode behavior and spectral splitting near the EP (second-order) in this system?
• RQ3 How does the Sagnac rotation signal modify the dual-SBL beating frequency near the EP, and can this be predicted quantitatively?
• RQ4 What are the practical limits to EP-enhanced sensing (noise, Kerr effects) in on-chip Brillouin gyroscopes?
• RQ5 Does the observed enhancement follow the predicted square-root dependence near the EP and saturate under real-world conditions?

Key findings

• The experiment shows rotation-rate transduction boosted by up to about 4× near the EP.
• A second-order EP is identified, arising from dissipative coupling between CW and CCW SBL modes.
• The SBL beating frequency follows the square-root dependence on pump detuning near the EP (Eq. 4) and matches measurements.
• The Sagnac transduction factor S agrees with the model and exhibits a near-EP enhancement proportional to Δω_p/√(Δω_p^2−Δω_c^2) (Eq. 6).
• Kerr-induced shifts are small and do not significantly affect the EP-enhanced sensitivity under balanced pumping.
• Thermal and pump fluctuations are the main noise sources that grow closer to the EP, but the enhancement trend remains evident.