[Paper Review] Fundamental charge noise in electro-optic photonic integrated circuits

Junyin Zhang, Zihan Li+4 more|arXiv (Cornell University)|Aug 29, 2023

Photonic and Optical Devices4 citations

TL;DR

This paper identifies fundamental charge noise—manifesting as $1/f^{1.2}$-scaled frequency fluctuations—in electro-optic photonic integrated circuits based on lithium niobate and tantalate. The noise arises from thermodynamically driven charge carrier density fluctuations, transduced via the strong Pockels effect, and emerges as the dominant noise source due to coherent cancellation of conventional thermo-refractive noise, establishing Johnson-Nyquist noise as the ultimate limit in Pockels-based photonics.

ABSTRACT

Understanding thermodynamical measurement noise is of central importance for electrical and optical precision measurements from mass-fabricated semiconductor sensors, where the Brownian motion of charge carriers poses limits, to optical reference cavities for atomic clocks or gravitational wave detection, which are limited by thermorefractive and thermoelastic noise due to the transduction of temperature fluctuations to the refractive index and length fluctuations. Here, we discover that unexpectedly charge carrier density fluctuations give rise to a novel noise process in recently emerged electro-optic photonic integrated circuits. We show that Lithium Niobate and Lithium Tantalate photonic integrated microresonators exhibit an unexpected Flicker type (i.e. $1/f^{1.2}$) scaling in their noise properties, significantly deviating from the well-established thermorefractive noise theory. We show that this noise is consistent with thermodynamical charge noise, which leads to electrical field fluctuations that are transduced via the strong Pockels effects of electro-optic materials. Our results establish electrical Johnson-Nyquist noise as the fundamental limitation for Pockels integrated photonics, crucial for determining performance limits for both classical and quantum devices, ranging from ultra-fast tunable and low-noise lasers, Pockels soliton microcombs, to quantum transduction, squeezed light or entangled photon-pair generation. Equally, this observation offers optical methods to probe mesoscopic charge fluctuations with exceptional precision.

Motivation & Objective

• To identify and characterize a previously unobserved noise mechanism in electro-optic photonic integrated circuits based on ferroelectric materials.
• To determine whether thermodynamical charge noise, previously theoretical, manifests in practical electro-optic microresonators.
• To assess the relative contributions of charge noise versus conventional thermo-refractive noise in determining the ultimate performance limits of integrated photonics.
• To establish the role of Johnson-Nyquist noise as the fundamental limit in Pockels-based integrated photonics for quantum and classical applications.
• To demonstrate that optical measurements can probe mesoscopic charge fluctuations with high precision using these devices.

Proposed method

• Employed balanced homodyne interferometry to measure intrinsic cavity frequency noise with sub-Hz resolution, using a high-finesse Fabry-Perot cavity as a phase reference.
• Conducted DC-shift measurements using a PDH-locked external cavity diode laser and a wavemeter to track resonance frequency fluctuations over 10 kHz–10 MHz bandwidth.
• Performed coherently driven nonlinear response measurements via intensity-modulated pump laser and sideband probing to extract coherent transfer functions.
• Used self-injection locking of a DFB laser to the microresonator and heterodyne beat note detection for high-resolution phase noise analysis.
• Applied vector network analysis to measure the complex response function of the cavity under test, enabling noise spectral density extraction.
• Theoretical modeling accounted for pyroelectric and electro-optic nonlinearities, showing coherent cancellation of thermo-refractive noise due to PyroEO effects.

Experimental results

Research questions

• RQ1 Does thermodynamically driven charge carrier density fluctuation give rise to measurable optical frequency noise in electro-optic microresonators?
• RQ2 How does the observed $1/f^{1.2}$-type noise in LiNbO3 and LiTaO3 microresonators compare to conventional thermo-refractive noise models?
• RQ3 To what extent do pyroelectric and electro-optic nonlinearities in ferroelectrics lead to coherent cancellation of thermo-refractive noise?
• RQ4 Can optical measurements of cavity frequency noise be used to probe mesoscopic charge fluctuations with high sensitivity?
• RQ5 What is the ultimate performance limit imposed by charge noise in Pockels-based integrated photonic devices?

Key findings

• Electro-optic microresonators in lithium niobate and tantalate exhibit a $1/f^{1.2}$-type noise spectrum across 10 kHz to 10 MHz, significantly deviating from standard thermo-refractive noise predictions.
• The observed noise is attributed to thermodynamical charge carrier density fluctuations transduced into refractive index fluctuations via the strong Pockels effect.
• Thermo-refractive noise is coherently canceled due to the combined pyroelectric and electro-optic nonlinearities (PyroEO effect), making charge noise the dominant noise source.
• The results establish Johnson-Nyquist noise as the fundamental limit for Pockels-based integrated photonics, affecting applications from ultrafast lasers to quantum transduction.
• The noise level observed is consistent with theoretical predictions of thermal-charge-carrier-refractive (TCCR) noise, previously undetected in optics.
• The findings open a path for using high-Q electro-optic microresonators as precision probes of mesoscopic charge dynamics in condensed matter systems.