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International Conference on Laser, Optics and Optoelectronic Technology (LOPET)

Pound-Drever-Hall locking of four-mirror optical cavity for cold atom interferometry

shaojun gao, guodong xin, qufei wang, Lingxiao Zhu, Xiaoxiao Ma

Published Aug 20, 2025• 0 citations
Publisher

Abstract

Atom interferometers, as precision gravity sensors, encounter inherent miniaturization challenges due to constraints like interference time and laser pulse intensity. Optical cavities address these limitations through wavefront purification, spatial mode filtering, and resonance-enhanced amplification, significantly enhancing interferometer stability and performance for compact quantum sensing applications. This study develops a fiber-integrated four-mirror cavity locking system based on the Pound-Drever-Hall (PDH) technique combined with modulation transfer spectroscopy (MTS), designed for cavityenhanced Raman resonance control in atom interferometry. The system utilizes a fiber-based electro-optic modulator (EOM) for red-detuned frequency shifting, with MTS stabilization applied to lock the cavity to the +1st-order sideband while maintaining the carrier as the locking beam. A fiber-optic circulator architecture in the PDH feedback loop eliminates environmental disturbances (stray light and vibrations) on dispersion-based error signals. Experimental implementation demonstrates continuous cavity locking for over 2 hours with transmitted power fluctuations below 2%. Validation using pulsed Raman beams (400μs duration) confirms the scheme's compatibility with cavity-enhanced atom interferometry, establishing a methodological framework for multi-beam quantum interference experiments in optical cavity.

DOI
10.1117/12.3075990

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arXiv:2409.08764•Sep 13, 2024•1 citation
Atom interferometers, as precision gravity sensors, encounter inherent miniaturization challenges due to constraints like interference time and laser pulse intensity. Optical cavities address these limitations through wavefront purification, spatial mode filtering, and resonance-enhanced amplification, significantly enhancing interferometer stability and performance for compact quantum sensing applications. This study develops a fiber-integrated four-mirror cavity locking system based on the Pound-Drever-Hall (PDH) technique combined with modulation transfer spectroscopy (MTS), designed for cavityenhanced Raman resonance control in atom interferometry. The system utilizes a fiber-based electro-optic modulator (EOM) for red-detuned frequency shifting, with MTS stabilization applied to lock the cavity to the +1st-order sideband while maintaining the carrier as the locking beam. A fiber-optic circulator architecture in the PDH feedback loop eliminates environmental disturbances (stray light and vibrations) on dispersion-based error signals. Experimental implementation demonstrates continuous cavity locking for over 2 hours with transmitted power fluctuations below 2%. Validation using pulsed Raman beams (400μs duration) confirms the scheme's compatibility with cavity-enhanced atom interferometry, establishing a methodological framework for multi-beam quantum interference experiments in optical cavity.
We present a cavity-enhanced optical absorption measurement technique based on high-bandwidth Pound-Drever-Hall (PDH) sideband locking, which naturally provides a real-time, resonant amplitude quadrature readout that can be mapped onto the cavity’s internal loss rate. Cavity-enhancement is widely used in the detection of small signals -- an optical cavity traps photons between two highly reflective mirrors, thereby enhancing the effective path length through the absorber [Romanini et al., 2014]. Most of the available techniques, however, probe transmission signals and/or do not provide the stability required for the detection of fast signals.To ensure optimal enhancement of absorption sensitivity, probe light must be locked on resonance with the cavity. PDH locking relies on phase modulation of a laser beam, creating two sidebands on the original carrier frequency [Black, 2001]. Upon reflection from a cavity, this modulated light produces a heterodyne signal (beat note) that can be detected and demodulated. We use the phase quadrature of the beat note to lock the cavity on resonance with one of the sidebands, achieving a locking bandwidth in the MHz range that keeps cavity and probe light resonant for hours [Reinhardt et al., 2017]. Simultaneously, we monitor the amplitude quadrature, which provides a continuous, real-time, heterodyne-amplified readout of the cavity’s internal absorption. Our PDH sideband cavity-enhanced absorption readout technique (SideCAR) ensures high sensitivity without reaching saturation limits in the intracavity medium by keeping a relatively low intracavity power, since only one of the sidebands is allowed to enter the cavity, but the signal is boosted above the detector noise by heterodyne detection (using the carrier as a local oscillator).This thesis presents the theoretical formulation of the sensing technique together with the expressions for the sensitivity limit (ultimately given by the laser quantum noise), and the experimental results from a test cavity setup. Probing a proof-of-concept 5-cm-long Fabry-Perot cavity with a coupled power of 160 uW, we measure an absorption sensitivity of 7*10^(-11) cm^(-1)/sqrt(Hz) at 100 kHz (roughly the cavity bandwidth), a factor of 11 from the shot-noise limit. This technique allows the detection of small, transient signals against absorptive backgrounds with sensitivity close to the shot noise limit using a reflection-based measurement in a fairly simple setup, opening the possibility of developing new microscopic single-port real-world sensors for transients, for example using micron-scale optical fiber cavities
Precise measurement of the resonance frequencies of microwave and optical cavities is a foundational capability across quantum technologies. One of the highest-performance methods currently available is Pound-Drever-Hall (PDH) spectroscopy, which relies on a three-tone interrogation and eliminates sensitivity to path-length fluctuations. However, PDH is known to suffer from systematic offsets due to residual amplitude modulation and demodulation-phase errors. Here we leverage an optimal linear combination of the phases of the three interrogation tones to implement a method for robust cavity metrology which eliminates significant systematic-error sensitivities of PDH. We experimentally demonstrate robust cavity measurements in both the optical and microwave frequency regime, showing insensitivity to sideband imbalance and demodulation phase, as well as demonstrating single-shot readout of a cavity-coupled superconducting qubit.
We have proposed an artificial modulation-free Pound-Drever-Hall (PDH) method for laser frequency stabilization and demonstrated it via two-color polarization spectroscopy of Rydberg electromagnetically induced transparency (EIT) resonance in a room-temperature rubidium vapor. Due to the unique error signal profile, the conventional PDH method owns a large capture range in laser frequency locking. Here we manually construct a PDH error signal via a linear combination of polarization spectroscopies of the Rydberg EIT resonances without and with a magnetic field applied. The artificial modulation-free PDH error signal owns a subnatural linewidth dispersion curve as well as a large capture range with which we successfully stabilize the laser to an absolute atomic frequency reference in a long running time, immune to environmental fluctuation and even manmade impulse perturbation. This method can provide an absolute frequency reference based on atomic transition while keeping similar locking ability to provide corrections for frequency fluctuations over a broad bandwidth as the conventional PDH.
Abstract We present a stabilization technique developed to lock and dynamically tune the resonant frequency of a moderate finesse Fabry-Pérot (FP) cavity used in precision atom-cavity quantum electrodynamics (QED) experiments. Most experimental setups with active stabilization either operate at one fixed resonant frequency or use transfer cavities to achieve the ability to tune the resonant frequency of the cavity. In this work, we present a simple and cost-effective solution to actively stabilize an optical cavity while achieving a dynamic tuning range of over 100 MHz with a precision under 1 MHz. Our unique scheme uses a reference laser locked to an electro-optic modulator (EOM) shifted saturation absorption spectroscopy (SAS) signal. The cavity is locked to the PDH error signal obtained from the dip in the reflected intensity of this reference laser. Our setup provides the feature to efficiently tune the resonant frequency of the cavity by only changing the EOM drive without unlocking and re-locking either the reference laser or the cavity. We present measurements of precision control of the resonant cavity frequency and vacuum Rabi splitting (VRS) to quantify the stability achieved and hence show that this technique is suitable for a variety of cavity QED experiments.
We present a stabilization technique developed to lock and dynamically tune the resonant frequency of a moderate finesse Fabry-Pérot (FP) cavity used in precision atom-cavity quantum electrodynamics (QED) experiments. Most experimental setups with active stabilization either operate at one fixed resonant frequency or use transfer cavities to achieve the ability to tune the resonant frequency of the cavity. In this work, we present a simple and cost-effective solution to actively stabilize an optical cavity while achieving a dynamic tuning range of over 100 MHz with a precision under 1 MHz. Our unique scheme uses a reference laser locked to an electro-optic modulator (EOM) shifted saturation absorption spectroscopy (SAS) signal. The cavity is locked to the PDH error signal obtained from the dip in the reflected intensity of this reference laser. Our setup provides the feature to efficiently tune the resonant frequency of the cavity by only changing the EOM drive without unlocking and re-locking either the reference laser or the cavity. We present measurements of precision control of the resonant cavity frequency and vacuum Rabi splitting (VRS) to quantify the stability achieved and hence show that this technique is suitable for a variety of cavity QED experiments.
The Pound--Drever--Hall (PDH) technique is routinely used to stabilize the frequency of a laser to a reference cavity. Electronic sideband (ESB) locking, a PDH variant, bridges the frequency gap between the discrete cavity resonances and a desired laser frequency. Here we use quadrature amplitude modulation (QAM), a standard technique in digital communications, to generate the high-quality phase-modulated radio-frequency (rf) drive required for ESB locking. We develop a theoretical framework to analyze how in-phase/quadrature-phase (I/Q) impairments distort the ESB error signal and induce frequency offsets relevant to ultranarrow-linewidth lasers. We then design and implement a direct software-defined radio (SDR) on an UltraScale+ RFSoC platform, frequently adopted across modern quantum-computing systems, to digitally compensate QAM I/Q impairments. Using this device, we generate phase-modulated rf signals with a large phase-modulation index of 1.011.011.01 rad and root-mean-square I/Q errors below 0.3 %0.3\ \%0.3 % over a carrier-frequency range of 350 MHz350~\mathrm{MHz}350 MHz to 1.75 GHz1.75~\mathrm{GHz}1.75 GHz. Finally, we lock a laser to an ultralow expansion (ULE) reference cavity and demonstrate continuous laser-frequency tuning by ramping the carrier frequency while maintaining lock, validating the continuous tunability of our ESB locking instrument.