Pound-Drever-Hall Research Articles

Cavity-enhanced light-induced thermoelastic spectroscopy for trace-gas sensing

We report a trace gas sensing technique of cavity-enhanced light-induced thermoelastic spectroscopy (CE-LITES) with a Fabry–Pérot (F-P) optical cavity and a high-quality-factor resonant quartz tuning fork (QTF). Pound-Drever-Hall (PDH) locking method is used to lock the laser to the optical cavity. With only a short 9-cm optical cavity (finesse, ∼1283) and a tiny commercially available standard QTF (Q-factor, ∼38910), a CE-LITES sensor for acetylene detection was developed to demonstrate this technique, achieving a minimum detection limit (MDL) of 7.5 parts-per-billion (ppb) for a 216-s integration time and a normalized noise equivalent absorption (NNEA) coefficient of as low as 1.91 × 10−10 cm−1·W·Hz−1/2. The developed technique bears promising potential for sensitive and accurate measurement that is needed for studying trace gas-phase analyte.

Pound–Drever–Hall feedforward: laser phase noise suppression beyond feedback

Pound–Drever–Hall (PDH) laser frequency stabilization is a powerful technique widely used for building narrow linewidth lasers. This technique is, however, ineffective in suppressing high-frequency (>100kHz) laser phase noise detrimental for many applications. Here, we introduce an effective method that can greatly enhance its high-frequency performance. The idea is to recycle the residual PDH signal of a laser locked to a cavity by feedforwarding it directly to the laser output field after a delay fiber. Using this straightforward method, we demonstrate a phase noise suppression capability about four orders of magnitude better than just using the usual PDH feedback for noise around a few MHz. We further find that this method exhibits noise suppression performance equivalent to cavity filtering. This method holds great promise for applications demanding highly stable lasers with diminished phase noise up to tens of MHz (e.g., precise and high-speed control of atomic and molecular quantum states).

High-accuracy optical vector network analyzer for optical notch and bandpass responses

A high-accuracy optical vector network analyzer (OVNA) based on optical carrier-suppressed double sideband (CS-DSB) modulation is proposed and experimentally demonstrated. The ±1st-order sideband signals are generated by CS-DSB modulation and then pass through the symmetric optical device under test (DUT). The band-stop or band-pass responses can be realized by detecting and processing the double frequency of the driven RF signal. Compared with the conventional symmetrical DSB-based OVNA, the measurement accuracy is improved by eliminating the errors caused by the even-order sidebands, and the complexity is reduced as the proposed method with only one step measurement can avoid the complex postprocessing. In addition, the optical carrier is aligned to the center frequency of the DUT by employing the Pound–Drever–Hall (PDH) technique, which provides stable measurement. At the same time, the limitation that the band-pass responses cannot be measured by the traditional single-sideband (SSB)-based OVNA is overcome. Additionally, accurate magnitude and phase responses of the DUT near the optical carrier can be also achieved since the proposed OVNA is optical filter-immune. The proposed method is theoretically analyzed and verified by experiment. A Fabry–Perot (FP) interferometer serves as the symmetric DUT; the band-stop responses in a frequency range of 6 GHz are obtained with a resolution of 1.2 MHz; and the band-pass responses with the range from 0 to 13 GHz offsetting the optical carrier are also obtained. The measurement time can reach up to 30 min with high stability. The proposed OVNA offers enhanced accuracy and a stable approach for applications in photonic systems and other innovations.

Label-free detection and profiling of individual solution-phase molecules.

Most chemistry and biology occurs in solution, in which conformational dynamics and complexation underlie behaviour and function. Single-molecule techniques1 are uniquely suited to resolving molecular diversity and new label-free approaches are reshaping the power of single-molecule measurements. A label-free single-molecule method2-16 capable of revealing details of molecular conformation in solution17,18 would allow a new microscopic perspective of unprecedented detail. Here we use the enhanced light-molecule interactions in high-finesse fibre-based Fabry-Pérot microcavities19-21 to detect individual biomolecules as small as 1.2 kDa, a ten-amino-acid peptide, with signal-to-noise ratios (SNRs) >100, even as the molecules are unlabelled and freely diffusing in solution. Our method delivers 2D intensity and temporal profiles, enabling the distinction of subpopulations in mixed samples. Notably, we observe a linear relationship between passage time and molecular radius, unlocking the potential to gather crucial information about diffusion and solution-phase conformation. Furthermore, mixtures of biomolecule isomers of the same molecular weight and composition but different conformation can also be resolved. Detection is based on the creation of a new molecular velocity filter window and a dynamic thermal priming mechanism that make use of the interplay between optical and thermal dynamics22,23 and Pound-Drever-Hall (PDH) cavity locking24 to reveal molecular motion even while suppressing environmental noise. New in vitro ways of revealing molecular conformation, diversity and dynamics can find broad potential for applications in the life and chemical sciences.

Analysis of alignment requirements for off-axis resonant cavity based on coupling efficiency

The optical path length stability of the off-axis four-reflection telescope is one of the key technical indicators for the TianQin gravitational wave detection system. In the mHz observation band, the telescope must exhibit an optical path length stability of 0.4pm/Hz1/2. As a feasible solution, the optical path length stability measurement of the off-axis four-reflection telescope based on the Pound–Drever–Hall (PDH) technique imposes stringent requirements on the alignment of the off-axis resonant cavity (ORC). Taking the off-axis two-reflection prototype as the research object, we propose a Monte Carlo analysis-based method for ORC alignment precision analysis. By considering misalignment as an intermediate function, we establish a relationship between the coupling efficiency of the ORC and the wavefront aberration of the telescope. The research results show that by considering the combined effects of multiple misalignment couplings of the primary and secondary mirrors, when the detected telescope wavefront aberration is better than 0.068λ (λ=1064nm) with a probability of 98%, the ORC coupling efficiency can achieve greater than 40% with a probability of 97.13%, which can be used as the main reference indicator for system misalignment analysis. This method simplifies the alignment difficulty of the target under test and can provide alignment reference for subsequent resonant cavities with internal off-axis telescopes.

Optimizing frequency noise calibration and manipulation in an active feedback control loop

Frequency noise is a key limitation factor for frequency sensitive ultra-high precision measurement. An excellent methodology for its calibration is the error signal extracting from a cavity length locking process via Pound–Drever–Hall (PDH) technique, and also can be applied as a noise discriminator for active feedback control. However, the stabilized laser frequency noise performance was poor to meet the applications for ultra-high precision measurement, and no literature has analyzed the cause in detail. In this Letter, we found that various of noise floors in the in-loop sensing and modulation depth in the PDH error signal should be responsible for the poor noise performance. Meanwhile, differing from the theoretical prediction (1.08), the modulation depth in frequency noise feedback controlling should be optimized by simultaneously considering the detector’s gain and saturation power, cavity impedance matching, and control loop gain. In our case, it was fixed to be 0.17. The experimental results confirm our theoretical analysis well. Finally, the stabilized laser frequency noises are reduced to 10−1Hz/Hz for out-of-loop and 3×10−2Hz/Hz for in-loop. The results demonstrate an efficient settlement solution for active frequency noise feedback control.

Synthetic FM triplet for AM-free precision laser stabilization and spectroscopy

The Pound–Drever–Hall (PDH) cavity-locking scheme has found prevalent uses in precision optical interferometry and laser frequency stabilization. A form of frequency modulation spectroscopy, PDH enjoys superior signal-to-noise recovery, large acquisition dynamic range, wide servo bandwidth, and robust rejection of spurious effects. However, residual amplitude modulation at the signal frequency, while significantly suppressed, still presents an important concern for further advancing the state-of-the-art performances. Here we present a simplified and improved scheme for PDH using an acousto-optic modulator to generate digital phase reference sidebands instead of the traditionally used electro-optic modulator approach. We demonstrate four key advantages: (1) the carrier and two modulation tones are individually synthesized and easily reconfigured, (2) robust and orthogonal control of the modulated optical field is applied directly to the amplitude and phase quadratures, (3) modulation synthesis, demodulation, and feedback are implemented in a self-contained and easily reproducible electronic unit, and (4) superior active and passive control of residual amplitude modulation is achieved, especially when the carrier power is vanishingly low. These distinct merits stimulate new ideas on how we optimally enact PDH for a wide range of applications.

Near infrared ammonia sensing system based on on‐axis cavity‐enhanced absorption spectroscopy using board‐level mode‐locked circuit

AbstractWe proposed a near‐infrared gas sensing system based on mode‐locked cavity‐enhanced absorption spectroscopy. A distributed feedback fiber laser (DFB‐FL) with a central wavelength of 1550 nm was employed as the light source, coupling with a Fabry–Perot (F‐P) cavity with an effective optical path length of 42.7 m served as the gas cell. By utilizing a homemade mode‐locked electric circuit and Pound–Drever–Hall (PDH) technique, the mode‐locking between the DFB‐FL and F‐P cavity was achieved with long‐term fluctuation of 3.2%. System performance was verified using ammonia as the analyte gas with a target line at 1550.17 nm. Allan variance analysis revealed that the minimum detection limit was 15.8 ppm with an averaging time of 216 s, corresponding to an absorption coefficient of 2.3 × 10−7 cm−1.

Quantum-enhanced optical phase tracking via squeezed state

Quantum-enhanced optical phase tracking is a quantum optical technique for tracking and measuring optical phases with high accuracy. It has important applications in laser interferometry, spectral analysis, and optical measurements. In this study, we propose a quantum-enhanced optical phase tracking protocol based on squeezed state optical fields. By using a continuous solid-state laser source with a central wavelength of 1064 nm, combing second harmonic generation, optical parametric oscillator, and PDH (Pound-Drever-Hall) locking technology, we prepare an initial squeezed state with a squeezing level of (8.0±0.2) dB. Through signal modulation technique and demodulation technique, we control the phase of the squeezed state optical field, thereby realizing the quantum-enhanced tracking of optical phases within the range of 0－2π. Compared with classical protocols, this protocol can suppress the noise fluctuations of phase tracking to at least 6.27 dB below the shot noise limit, improving the phase tracking accuracy by more than 76.4%. Because of the high requirements for phase measurement accuracy in applications such as angle estimation, phased array radar, and phased array sonar, this protocol is expected to improve the phase estimation accuracy beyond the shot noise limit. It provides compressed light sources for relevant fields, laying a theoretical and experimental foundation for higher-precision spatial positioning and quantum ranging techniques. The probe is made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid residues. The sequence of amino acids in a protein is determined by a gene and encoded in the genetic code. This can happen either before the protein is used in the cell, or as part of control mechanism.

Injection-seeded high-power Yb:YAG thin-disk laser stabilized by the Pound-Drever-Hall method.

We demonstrate an injection-seeded thin-disk Yb:YAG laser at 1030 nm, stabilized by the Pound-Drever-Hall (PDH) method. We modified the PDH scheme to obtain an error signal free from Trojan locking points, which allowed robust re-locking of the laser and reliable long-term operation. The single-frequency pulses have 50 mJ energy (limited to avoid laser-induced damage) with a beam quality of M2 < 1.1 and an adjustable length of 55-110 ns. Heterodyne measurements confirmed a spectral linewidth of 3.7 MHz. The short pulse build-up time (850 ns) makes this laser suitable for laser spectroscopy of muonic hydrogen, pursued by the CREMA collaboration.

Laser frequency stabilization based on Fano resonance in a microcylinder cavity.

We investigate the application of Fano resonance in microcylinder cavities for laser frequency stabilization. By combining Fano resonance and the differential subtraction method, we successfully reproduce the error signal of the traditional Pound-Drever-Hall (PDH) technique. The frequency noise of the laser, when locked to both microsphere and microcylinder cavities, approaches the thermal noise limit. The microcylinder cavity, with a high Q factor of ∼108, benefiting from its large mode volume, exhibits a significant reduction in frequency noise by one order of magnitude compared with a microsphere in the frequency range of 0.1 to 10 kHz, achieving a minimum noise of ∼2.25 Hz2/Hz at 10 kHz. As this approach eliminates the need for additional electronic circuits typically used in the PDH technique, it holds promise as a cost-effective and reliable solution for laser frequency stabilization.

Low phase noise operation of a cavity-stabilized 698 nm AlGaInP-based VECSEL.

We report for the first time a high performance, single frequency AlGaInP-based VECSEL (vertical-external-cavity surface-emitting-laser) with emission at 698 nm, targeting the clock transition of neutral strontium atoms. Furthermore, we present comprehensive noise characterization of this class-A semiconductor laser, including the residual fast phase noise in addition to the frequency and relative intensity noise. The low noise VECSEL has output power at around 135 mW with an estimated linewidth of 115 Hz when frequency stabilized via the Pound-Drever-Hall (PDH) technique to a high finesse reference cavity, without intermediate stabilization. The phase noise is measured to be below -126 dBc/Hz for frequencies between 10 kHz and 15 MHz with a total integrated phase noise of 3.2 mrad, suitable not only for ultra-cold neutral strontium-based quantum technologies, such as optical clocks, but also with potential for atom-interferometry applications.

Carrier-envelope offset frequency measurement by means of an external optical resonator

A general-purpose method based on the implementation of the asymmetric Pound–Drever–Hall (PDH) technique is proposed to measure the carrier-envelope offset (CEO) frequency of a mode-locked laser using an external optical cavity. By analyzing the synchronously demodulated signal of the spectrally filtered cavity reflection when the optical resonator is locked to the mode-locked laser, a discriminating signal depending on the relative frequency offset between the mode-locked and optical cavity comb-like spectra is obtained. For a given geometry and group delay dispersion (GDD) of the cavity parameters (i.e., a known cavity mode offset), this signal can be used to retrieve the laser CEO. This approach turns out to be advantageous in terms of setup complexity with respect to other well-known techniques that rely on non-linear frequency generation, such as f–2f interferometers. In addition, this method can be used to precisely determine the laser–cavity spectral coupling, which is an important topic in cavity-enhanced spectroscopy and non-linear optics applications. After the theoretical description of the generalized asymmetric PDH signal, an experimental validation of the proposed method is reported using an Er-doped fiber frequency comb source centered at 1,550 nm, with a repetition rate of 250 MHz, locked to a linear optical cavity with a 1 GHz free spectral range. The theoretical effect of the GDD is confirmed experimentally using different cavity configurations. Moreover, the comparison with the CEO frequency values measured using an f–2f interferometer demonstrates the feasibility of the proposed method.

Study on Acoustic Sensing Characteristics of a Novel Grooved SiO2 Waveguide Resonator

Photoacoustic sensor has become a research hotspot in the field of acoustic sensing in recent years due to its high sensitivity and anti-electromagnetic interference. A novel high- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> grooved silica waveguide acoustic sensor (GSWAS) is proposed, which is etched and filled the Polydimethylsiloxane (PDMS) above the waveguide coupling region. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> factor of the sensor is of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.82\times 10^{{6}}$ </tex-math></inline-formula> . In order to detect the sound signal easily and intuitively, an acoustic sensing system was built by using Pound–Drever–Hall (PDH) frequency locking technology, and the weak sound signal was demodulated by using the method of synchronous demodulation of high frequency modulation. Experimental results show that the designed sensor has a flat response in the frequency range of 0.4–15 kHz and can accurately detect the acoustic signal, with a high sensitivity of 1143 mV/Pa and a signal-to-noise ratio (SNR) of up to 45.19 dB under the action of acoustic signal of 8 kHz. This article explores a new method of frequency bandwidth and high sensitivity sound detection, laying a foundation in the field of acoustic signal detection such as aeroacoustics, photoacoustic imaging, and all that.

The Application of Pound–Drever–Hall Technology in High-Resolution Sensing—A Review

Pound–Drever–Hall (PDH) technology is a frequency stabilization technology, which is widely used in laser frequency stabilization systems. The high-frequency stability in this technology has attracted the attention of sensor researchers. In this article, we review the basic principle of PDH technology and introduce the related improved PDH technology. In view of the wide application of PDH technology in high-resolution sensors in recent years, we will introduce the sensors involved in this technology in detail according to the core optical sensing components and their main features. Furthermore, we summarize the physical quantities suitable for high-resolution sensing using PDH technology. The problems and prospects of PDH in high-resolution sensors are presented at the conclusion to assist researchers in clarifying their research directions. This article presents an in-depth analysis of the current state of research regarding PDH-related high-resolution sensor technologies and makes a prognosis regarding the future direction of the high-resolution sensor, which can improve the sensing performance in many areas.

High-Resolution Optical Fiber Strain Sensor Array Based on Pound-Drever-Hall Technique With Fast Phase Regulation Method

Sensors that use Pound-Drever-Hall (PDH) technology benefit from the good performance and high resolution but are in shortage of large-scale multiplexing. This paper proposed a fast phase regulation method for the hybrid PDH multiplexing scheme based on space division multiplexing and time division multiplexing. Using both the information provided by an I/Q demodulator, the points from the error signal that the close-loop control requires can be rebuilt in the software with a phase rotation algorithm within a time period of less than 20 us. The relationship between the multiplexing scale and parameters selection is also given in this paper, showing that the system can have a larger multiplexing scale up to several tens sensors with appropriate parameters selection. A multi-channel strain array of up to 10 channels with a resolution better than 10 pϵ/√Hz@1 Hz over 50 Hz band per channel has been realized in this paper, and the system works with good channel crosstalk isolation. The experiment results show that the developed instrument has potential usage in the field of large-scale quasi-static strain measurement and earthquake observation.

Compact sub-hertz linewidth laser enabled by self-injection lock to a sub-milliliter FP cavity

A narrow linewidth laser (NLL) of high frequency stability and small form factor is essential to enable applications in long-range sensing, quantum information, and atomic clocks. Various high performance NLLs have been demonstrated by Pound-Drever-Hall (PDH) lock or self-injection lock (SIL) of a seed laser to a vacuum-stabilized Fabry-Perot (FP) cavity of ultrahigh quality (Q) factor. However, they are often complicated lab setups due to the sophisticated stabilizing system and locking electronics. Here we report a compact NLL of 67-mL volume, realized by SIL of a diode laser to a miniature FP cavity of 7.7 × 108 Q and 0.5-mL volume, bypassing table-size vacuum as well as thermal and vibration isolation. We characterized the NLL with a self-delayed heterodyne system, where the Lorentzian linewidth reaches 60 mHz and the integrated linewidth is ∼80 Hz. The frequency noise performance exceeds that of commercial NLLs and recently reported hybrid-integrated NLL realized by SIL to high-Q on-chip ring resonators. Our work marks a major step toward a field-deployable NLL of superior performance using an ultrahigh-Q FP cavity.

Stability of silicon resonator temperature sensors with the Pound-Drever-Hall technique.

In this paper, we research the temperature stability of silicon-based ring resonator thermometers utilizing the Pound-Drever-Hall (PDH) technique. A slight temperature fluctuation of 12.2 mK in 200 s was experimentally detected by immersing the sensor in the triple point of water (TPW) system with ultrahigh precision. Additionally, factors that affect temperature stability, including fundamental thermal noise, laser frequency drift, and power fluctuation were analyzed and calculated theoretically. This shows high consistency with experimental results. Moreover, it is proved that the laser drift can be suppressed from 11.3 pm to 0.013 pm with the developed experimental system based on the PDH technique. The silicon-based ring resonator as a potential platform for precise temperature monitoring is proved based on this work.

Passive resonant laser gyroscope with improvement of the suppression of residual amplitude modulation effects through correlation analysis

Passive resonant gyroscopes (PRGs) utilizing the Pound-Drever-Hall (PDH) method are expected to have a lower shot-noise limit in rotational rate measurement. However, there are also many technical obstacles in the way to touch the intrinsic noise floor. In this article, we find that the residual amplitude modulation (RAM) effect in the phase locking process deteriorates the accuracy and stability of the output signal of PRGs, and becomes a dominant limiting factor, especially in the low-frequency range. Since the suppression is not good enough in the PRGs due to the strict requirements, we propose to use the Pearson correlation coefficient to evaluate the correlation between the RAM signal and the PDH signal to achieve a sufficient correlation before active servo control. With the implementation of this improvement, the RAM-induced fractional frequency stability for the 12 m optical ring cavity is reduced down to . The contribution of the RAM effect to the rotational sensitivity of our 3 m × 3 m PRG is lowered to 8.0 × 10−9 rad s−1 at 1 mHz, which is one order of magnitude better than the unconstrained situation. With this upgrade, a rotational resolution of rad s−1 of the PRG is achieved, which is the best performance among all PRGs so far.

Pound-Drever-Hall locking scheme free from Trojan operating points.

The Pound-Drever-Hall (PDH) technique is a popular method for stabilizing the frequency of a laser to a stable optical resonator or, vice versa, the length of a resonator to the frequency of a stable laser. We propose a refinement of the technique yielding an "infinite" dynamic (capture) range so that a resonator is correctly locked to the seed frequency, even after large perturbations. The stable but off-resonant lock points (also called Trojan operating points), present in conventional PDH error signals, are removed by phase modulating the seed laser at a frequency corresponding to half the free spectral range of the resonator. We verify the robustness of our scheme experimentally by realizing an injection-seeded Yb:YAG thin-disk laser. We also give an analytical formulation of the PDH error signal for arbitrary modulation frequencies and discuss the parameter range for which our PDH locking scheme guarantees correct locking. Our scheme is simple as it does not require additional electronics apart from the standard PDH setup and is particularly suited to realize injection-seeded lasers and injection-seeded optical parametric oscillators.