We introduce optical feedback frequency stabilized cavity ring-down spectroscopy (OFFS-CRDS), a near-shot-noise-limited technique that combines kilohertz resolution with an absorption detection sensitivity of 5×10(-13) cm(-1) Hz(-1/2). Its distributed feedback laser source is stabilized to a highly stable V-shaped reference cavity by optical feedback and fine-tuned by means of single-sideband modulation. The stability of this narrow laser is transferred to a ring-down (RD) cavity using a new fibered Pound-Drever-Hall (PDH) locking scheme without a dedicated electro-optic phase modulator, yielding several hundred RD events per second. We demonstrate continuous coverage of more than 7 nm with a baseline noise of 5×10(-12) cm(-1) and a dynamic range spanning six decades. With its resonant intracavity light intensity on the order of 1 kW/cm2, the spectrometer was used for observing a Lamb dip in a transition of carbon dioxide involving four vibrational quanta. Saturating such a weak transition at 160 μW input power, OFFS-CRDS paves the way to Doppler-free molecular overtone spectroscopy for precision measurements of hyperfine structures and pressure shifts.
We introduce optical feedback frequency stabilized cavity ring-down spectroscopy (OFFS-CRDS), a near-shot-noise-limited technique that combines kilohertz resolution with an absorption detection sensitivity of 5×10(-13) cm(-1) Hz(-1/2). Its distributed feedback laser source is stabilized to a highly stable V-shaped reference cavity by optical feedback and fine-tuned by means of single-sideband modulation. The stability of this narrow laser is transferred to a ring-down (RD) cavity using a new fibered Pound-Drever-Hall (PDH) locking scheme without a dedicated electro-optic phase modulator, yielding several hundred RD events per second. We demonstrate continuous coverage of more than 7 nm with a baseline noise of 5×10(-12) cm(-1) and a dynamic range spanning six decades. With its resonant intracavity light intensity on the order of 1 kW/cm2, the spectrometer was used for observing a Lamb dip in a transition of carbon dioxide involving four vibrational quanta. Saturating such a weak transition at 160 μW input power, OFFS-CRDS paves the way to Doppler-free molecular overtone spectroscopy for precision measurements of hyperfine structures and pressure shifts.
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
\nWe present a simple, high-duty-cycle, cavity-enhanced optical absorption measurement technique based on delay-limited Pound-Drever-Hall (PDH) sideband locking. The chosen circuit naturally provides realtime readout of the amplitude quadrature of the PDH error signal, which can be mapped onto the cavity’s internal loss rate while using the phase quadrature to lock sideband frequency to the cavity mode. Our proofof-concept device comprises a 5-cm-long Fabry-Perot cavity with a 450 kHz bandwidth (finesse 6800, 350 ns power ringdown), and a feedback bandwidth of several MHz, limited primarily by the group delay of our electronics. This technique could readily be applied to other optical resonators such as fiber cavities, with potential applications in radiation dosimetry.\n
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 report what we believe to be the first systematic study of Doppler-free, nonlinear absorption by use of cavity ringdown spectroscopy. We have developed a variant of cavity ringdown spectroscopy for the mid-infrared region between 9 and 11 microm, exploiting the intracavity power buildup that is possible with continuous-wave lasers. The infrared source consists of a continuous-wave CO2 laser with 1-mW tunable infrared sidebands that couple into a high-finesse stable resonator. We tune the sideband frequencies to observe a saturated, Doppler-free Lamb dip in the nu7, 11(1,10) <-- 11(2,10) rovibrational transition of ethylene (C2H4). Power studies of the Lamb dip are presented to examine the intracavity effects of saturation on the Lamb-dip linewidth, the peak depth, and the broadband absorption.
We present a simple, continuous, cavity-enhanced optical absorption measurement technique based on high-bandwidth Pound-Drever-Hall (PDH) sideband locking. The technique provides a resonant amplitude quadrature readout that can be mapped onto the cavity’s internal loss rate and is naturally compatible with weak probe beams. With a proof-of-concept 5-cm-long Fabry-Perot cavity, we measure an absorption sensitivity ∼ 10 − 10 cm − 1 / Hz from 30 kHz to 1 MHz, and a minimum value of 6.6 × 10 − 11 cm − 1 / Hz at 100 kHz, with 38 µW collected from the cavity’s circulating power.
We present a simple, high-duty-cycle, cavity-enhanced optical absorption measurement technique based on delay-limited Pound-Drever-Hall (PDH) sideband locking. The chosen circuit naturally provides realtime readout of the amplitude quadrature of the PDH error signal, which can be mapped onto the cavity’s internal loss rate while using the phase quadrature to lock sideband frequency to the cavity mode. Our proofof-concept device comprises a 5-cm-long Fabry-Perot cavity with a 450 kHz bandwidth (finesse 6800, 350 ns power ringdown), and a feedback bandwidth of several MHz, limited primarily by the group delay of our electronics. This technique could readily be applied to other optical resonators such as fiber cavities, with potential applications in radiation dosimetry.