Photonic electrometry using a piezoelectric-Pockels microresonator

TL;DR

This work demonstrates antenna-free photonic electrometry in a LiNbO3 microresonator driven by a fixed-frequency semiconductor laser. By locking a resonant mode to the laser via electrooptic feedback and reading out the rf response with a PDH error signal, the authors show high-resolution E-field sensing that remains robust at signal frequencies beyond the optical resonance bandwidth, provided laser frequency noise dominates. A pronounced enhancement occurs at the microresonator's piezoelectric resonances around ~4 MHz, improving the resolution by about a factor of 3 to ~34 mV/(m·√Hz). The results indicate that cost-effective, compact laser sources can be used for high-performance photonic electrometry and pave the way for integration into chip-scale platforms and broader photonic sensing applications.

Abstract

Facilitated by low-noise laser frequency locking, optical microresonators with the Pockels effect have shown unprecedented high resolutions in sensing electrical field. However, the requirement for tunable and low-noise laser sources considerably increases the cost and the size of the system, thereby limiting the industrial applicability of the microresonator-based technology. Here, we explore the possibility of using a low-cost fixed-frequency semiconductor laser as the pump laser to perform radiofrequency electrometry. A resonant mode in a lithium niobate microresonator is frequency-locked to the laser using the electrooptic effect. This same effect also underlies the radiofrequency electric-field sensing mechanism. Our experimental results show that the electrometry resolution can be maintained at signal frequencies beyond the optical resonance bandwidth and that the signal-to-noise ratio does not change with varied coupling conditions as long as the laser frequency noise is the dominant noise source of the system. In addition, narrowband electrooptic sensitivity enhancement is observed at frequencies of the microresonator's piezoelectric resonances, resulting in a resolution enhancement factor of approximately 3 at signal frequencies around 4 MHz. Our work advances the photonic resonant electrometry technology by studying the bandwidth limitation, and opens the road to the employment of low-cost lasers in high-resolution sensing applications.

Figures (9)

• Figure 1: (a) Main diagram of the experimental setup. The angular frequency $\omega_\mathrm{m}$ is the laser-phase-modulation frequency. The frequency of the rf signal applied to the microresonator is denoted as $\omega_\mathrm{s}$. (b) The detailed configuration of the whispering-gallery-mode resonator. The dimensions of the microresonator and the electrodes are shown. (c) The transmission spectrum of a resonance with a $Q$ of approximately $1\times10^8$ and the corresponding PDH error signal. (d) Simulated electrode efficiency. The red-circled oval indicates the location of the optical mode. The inset shows the photo of the microresonator. (e) Laser-swept transmission spectrogram of an electrooptically modulated resonance, showing the sidebands generated by the frequency modulation of the resonance. The magnitude of the sidebands becomes abruptly strong around the modulation frequency of 4 MHz due to piezoelectric resonances of the microresonator. (f) Electrooptic modulation coefficients derived with the spectrogram in (e). The inset is the electrooptic modulation response profile of the microresonator measured by a VNA.
• Figure 2: (a) Electrometry sensitivity measured with the in-loop PDH error signal. The inset plots the ratio of the measured sensitivity to the electrooptic modulation coefficient displayed in Fig. \ref{['fig1']} (f), showing the cavity filtering effect. (b) Noise floor of the PDH signal. The inset shows the power spectral density (PSD) of the voltage of the in-loop PDH error signal around signal frequency of 1 MHz. (c) Electrometry resolution at varied signal frequencies.
• Figure 3: Noise spectra of system. Because the adopted self-heterodyne setup can only measure the laser noise at frequencies below offset frequency of 1 MHz, above this offset frequency we assume that the noise exhibits a white PSD profile as the flat dashed line. The spectra of the in-loop PDH signal and the laser amplitude noise show rises close to the offset frequency of 10 MHz, which is caused by the PDH phase modulation at 10 MHz.
• Figure 4: ESA-measured signal spectra with different coupling conditions. The signal frequencies are 0.5 MHz for (a) and 1 MHz for (b), respectively.
• Figure 5: (a) Dynamic range measured at discrete signal frequencies between 1 and 10 MHz. Error bars represent the standard deviation of 5 repeated measurements. (b) The measured PDH signal response to an applied rf signal at 5 MHz with gradually increased amplitude. A dynamic range of 67 dB is obtained with this take.